Follow up question: When we think about the question why Britain and not Germany, do we have any evidence to suggest that the legacy of the long dead but still popular Germanicus played a role?

Would a campaign in Germany, with Claudius inevitably claiming victory and taking the name Germanicus just have reminded everyone in Rome that Claudius was never supposed to succeed Tiberius?

Because they tried once, but failed to subdue the Germanic tribes.

The conquest of Gaul by Julius Caesar had left the Romans with a long border along the Rhine. The Celts in Gaul had been becoming more urbanized, living in walled towns called Oppidum. This was important to Caesar's campaign, because the towns friendly to him offered a base of operations and of support, the surplus agriculture supplying his legions. Caesar notes that at least one town had a senate. The Gauls were very much tied to their towns, although they tried abandoning some of them in the later stages of the revolt. This gave Caesar places in which he could force the Gauls to battle, or beseige them. Augustus then made a concerted effort to romanize the Gauls, bringing Roman-style law along with Roman luxuries.

(While I use "Celts" and "Germani" here, it's never clear whether any particular group spoke celtic or germanic languages. I also try to avoid "Germans" and "Germany", to avoid confusing the modern with the ancient.)

The classic image, promoted by Caesar himself in his writings, was of more civilized Gallic Celts to the west of the Rhine, and uncivilized Germanic tribes to the East. This is an oversimplification. The Germanii weren't nomadic tribes, but they lacked the centralization of the Gauls. As you move east of the Rhine and north of the Danube (and particularly north of the Main), the archaeological record of Oppida becomes more scarce, and there is more evidence of smaller villages, usually no more than 15 longhouses. While the Germans certainly farmed, they do not seem to have much surplus, nor a centralized system to collect it. They were much more willing to abandon settlements than their Gallic neighbors. The tribal confederation known as the Marcomanni, for example, seems to have picked up everything and moved somewhere around Bohemia, rather than come under Roman control. In Clauzwitz's terms, the oppida were a center of gravity for the Romans to attack...the Germani had no such weakness.

One thing about the Germani is they had a very warlike tradition, and raiding their neighbors was a way of life, a way not only to acquire wealth, but bring up the younger generation and occupy idle hands. Hubert (The Greateness and Decline of the Celts) suggests the tribal confederations, and therefore these raids became larger in the last century BC, putting increasing pressure on the Celtic tribes, and some of the Celtic settlements and oppida in Germania were abandoned around that time. This tradition was not appreciated by the Gauls, and they turned to their new master, the Romans, for help. The Romans themselves viewed the raids as criminal banditry (latronicum), a violation of Roman borders, and a threat to order in Gaul. Warriors from three of the Germanic tribal confederations, the Sugambri, Usipetes, and Tencteri, crossed the Rhine in 17 BC, conducting what must have been a large raid. The governor of Gaul, Marcus Lollius, marched to intercept the raiders. According to Dio, his “Roman cavalry” was ambushed, and put to flight, but it is unclear whether this was the legion’s own cavalry, or auxiliary cavalry. Legio V Alaudae (The Larks), then encountered the raiders and lost their eagle standard. Not much is known about this battle other than the Legion and Lollius survived, and the Germani withdrew as Lollius gathered additional forces, and Augustus arrived from Rome to personally oversee operations.

In 19 BC, Augustus completed the conquest of Hispania. This left him with several legions free for other operations. Starting in 16 BC, the Romans, led by Augustus’ stepsons, Nero Claudius Drusus and Tiberius Claudius Nero, began operations out of Gaul. They first secured the passages through the Alps in Raetia (now parts of Switzerland). In 12 BC, the Germanic tribes again crossed the Rhine, but were defeated by Drusus. The following year, the conquest of Germania began.

With little surplus agriculture, the Romans were forced to run supply lines back to Gaul, and the terrain was not very helpful. Tacitus describes Germania as covered in forests and swamps, productive in grain, with flocks and herds. Already by his time, much of the ancient forests had been cleared, but even today thirty percent of Germany is forests and woodlands. Extensive land reclamation starting in the Ninteenth Century has drained many of the swamps in Germany, but in this time they were quite extensive. Obviously the entire country wasn't as Tactius describes, but if the Romans traveled any distance into Germania, they would encounter chokepoints where swamps, hills, and forests narrowed their options. The Germani repeatedly ambushed Roman columns in these places. The other option, which the Romans were quite proficient, was to use the rivers. Here again, their options were limited. Only four navigatable rivers reach from the Rhine into Germany: The Main, Ruhr, Lahn, and Lippe, and none but the Main reach very far east. Reaching the Wesser and Elbe rivers required traversing the Wadden Sea, that portion of the North Sea along the north German coast. The North Sea storms made that a near impossibility in winter, and risky even in other times. The Oder was completely out of the question...it empties into the Baltic, requiring a trip around Jutland. This meant that the Romans had to winter along the Rhine and Lippe, returning from campaigns farther east.

So, the Germani had no 'center of gravity' which the Romans could attack. They could always pull up their villages and withdraw beyond the Elbe. They couldn't support the occupation force, and the Roman logistics were problematic. It's probable that Augustus underestimated the size of Germania when he invaded, thinking there was an ocean to the east connected to the saline Caspian Sea. But they found the German plains and forests were effectively endless. Still, the Romans temporarily pacified the area between the Wesser and Rhine. Attempts at romanization were made, notible at Waldgirmes, where a town and forum was established near the Main. In AD 6, Augustus had eight legions ready to make a pincer movement against the Marcomanni. This was to be the largest operation in Germania yet. The offensive was stopped when the revolt in Illyricum started. From the details of Augustus' response, the revolt must have been tremendously costly for the Romans. New taxies and levies were placed on Roman citizens, slaves were requisitioned, freed, and formed into units. This likely sapped Roman strength in Germania, and probably encouraged the Germans to revolt.

In AD 9, just as the Tiberius was finishing off the revolt, three Roman legions, under command of Publius Quinctilius Varus, were massacred at Teutoburgiensi Saltu (Either "Teutoburg Forest" or "Teutoburg Narrows"). The most likely location is Kalkriese, near Osnabrück. Here, a large marsh to the north comes near a large hill, making a perfect spot for an ambush. Very few excaped. The fort at Aliso (possibly modern Haltern) was beseiged, and it's garrison eventually escaped to the Rhine. The three legions (presumed to be XVII, XIIX, and XIX) were never reformed. Two more legions (probably Legio I Germania and Legio V Alaudae) remained to defend the Rhine under Varus' nephew, Lucius Nonius Asprenas. Tiberius hurried to reinforce the border, gathering the two legions still in Raetia (XVI Gallica and XXI Rapax), and more hastily recruited cohorts. There were more forced conscriptions in Italy to provide manpower. These were later further reinforced with four more legions, for a total of eight.

This large army fell under the command of Drusus' son, now adopted by Tiberius, Germanicus Julius Caesar. From AD 14 to AD 16, Germanicus repeatedly launched attacks into Germania, destroying any settlements he could find and recapturing one of the eagles from the lost legions. The Romans again faced repeated ambushes where the terrain bottlenecked his troops. Germanicus' brutal campaigns against the Marsi and the Chatti, and likely any Cherusci settlements he came across, were probably intended to anger the Germani enough that they would be willing to risk open battle to put a stop to the Roman incursions. If so, the plan worked, and the Germanic tribes formed up near the Weser river at a place named Idisiaviso. Germanicus' victory was near total, but other tribes attempted yet another ambush on the march back to the Rhine. Germanicus returned to Tiberius, now emperor, and requested another season to complete his reconquest. Tiberius refused, and sent Germanicus east.

Tiberius saw no hope of ever returning the province to Roman control, and had changed the Roman policy towards the Germani to one of diplomatic manipulation. Augustus' reported deathbed command to Tiberius to keep within the current borders of the empire may have been an invention of Tiberius to lay the responsibility for abandoning Germania on Augustus and not himself. Furthermore, Germanicus was much more popular than the dour Tiberius, and Tiberius may not have trusted his heir with eight legions that might march on Rome.

South-east Britannia was much like Gaul. That is, the people were settled into fortified towns. Their agriculture produced a surplus. They had a settled noble class that could be enticed with Roman luxuries. There were already Roman client kingdoms on the island. The Romans could move in, co-op the nobility, and let the locals run the day-to-day things. They could threaten the towns, forcing the Britons to battle. Furthermore, Claudius needed a quick victory, to secure himself as emperor. The lead/silver mines known to exist there were also an attractive target. And, of course, the pretext of the invasion was to return Verica, former king of the Atrebates and a Roman client, back to power.