A fine morning for an invasion - bright and clear, with a fine salt breeze whistling over the waves. The swift Gallic boats sweep steadily forward, stroke by stroke, toward the strange white cliffs, sending up plumes of spray when they catch a swell crosswise. The Roman soldiers on the decks stare resolutely forward, beads of moisture glistening on their armor.

[ominous music; camera moves over waves toward cliffs]

On the clifftops, barbarians are waiting. Sunlight glistens on spears. The breeze stirs a warrior's streaming hair. A horse hitched to a war chariot nickers...

There was something more than a little cinematic about Caesar's first sight of Britain: a substantial native force, amply supplied with photogenic war chariots, arrayed along the Cliffs of Dover, waiting. That they were ready for Caesar, however, is less surprising than you might think.

Southeastern Britain had close cultural and economic ties with the northwestern Gaul. As Caesar himself notes (BG 2.4, 5.12), the parts of Britain closest to the Continent had been occupied for decades by Gallic invaders. These relative newcomers maintained close contacts with their cousins on the other side of the Channel, and traded frequently with them. Caesar questioned some of these cross-channel merchants when preparing his expedition (4.20). He claims that he was unable to learn from them "what was the size of the island, nor what or how numerous were the nations which inhabited it, nor what system of war they followed, nor what customs they used, nor what harbors were convenient for a great number of large ships" (4.20). Although it is true, as Caesar himself notes elsewhere (5.13) that most Gallic merchants only dealt with the southeastern corner of Britain, the claim that the merchants could tell him nothing about British harbors or warfare is probably more than a little disingenuous, and intended to enhance the remoteness and savagery of Britain in the imagination of his Roman audiences. The remoteness of Britain was something of a trope in Roman literature.

Southeastern Britain, in short, was not El Dorado. The merchants Caesar questioned probably gave him a functional understanding of the harbors and political organization of southeastern Britain, and he soon supplemented this knowledge by sending his lieutenant Volusenus to reconnoiter the coast in a galley (4.21). By this point, thanks to talkative merchants, his planned invasion was common knowledge along the length of the British coast, as Caesar himself notes (4.21). Chieftains throughout southeastern Britain were on high alert; and once they sighted Volusenus' galley cruising back and forth near the most obvious harbors, it became clear that war was imminent. They did not know the hour and the day Caesar's troops would arrive - but they knew they were coming, and soon. The Britons that Caesar sighted atop the White Cliffs had probably been stationed in the vicinity, patrolling the most likely landing places, for several weeks.

We shouldn't imagine a large and cohesive army, but rather a series of relatively small and mobile groups stationed on prominent hills (4.23). Once Caesar's fleet was sighted, these forces began to coalesce, trying to anticipate where the Romans would land. When Caesar finally selected a landing place, the Britons -who had been shadowing his ships all day with the cavalry and chariots - were ready for him. They had been waiting for a long time.

We unfortunately do not have any Gallic sources so most of our information comes from Caesar himself. He does answer these questions but without much elaboration. Caesar is not writing as a chronicler or an anthropologist. He is writing in order to justify and glorify his own actions in Gaul to his Roman audience. We must always keep this intent in mind when reading his work.

 

The answer to your first question, how they knew he was coming, is answered by Caesar as follows:

In the mean time, his [Caesar's] purpose having been discovered, and reported to the Britons by merchants, embassadors come to him from several states of the island,

^De ^Bello ^Gallico, ^IV, ^21

The merchants trading across the Channel warned the Britons of Caesar's plans. There was a lot of contact between the British isles and the mainland. For example, Caesar informs us about a Gallic king from the mainland who in recent years also ruled over (parts of) Britain:

Divitiacus, the most powerful man of all Gaul, had been king; who had held the government of a great part of these regions, as well as of Britain ^De ^Bello ^Gallico, ^II, ^4

And he states that the Gauls living along Britain's shore were (relatively recent) arrivals from the mainland, which the archaeological record bears out:

the maritime portion [is inhabited] by those who had passed over from the country of the Belgae for the purpose of plunder and making war; almost all of whom are called by the names of those states from which being sprung they went thither, and having waged war, continued there and began to cultivate the lands. ^De ^Bello ^Gallico, ^V, ^12

Crucially, Caesar also claims that Gauls from across the Channel had aided his enemies during the fighting in mainland Gaul:

[Caesar] nevertheless resolved to proceed into Britain, because he discovered that in almost all the wars with the Gauls succors had been furnished to our enemy from that country;

^De ^Bello ^Gallico, ^IV, ^20

This is the most important claim Caesar makes about contact between the mainland and Britannic Gauls. He states here that his reason for invading Britain is the aid given to his enemies in Gaul by other Gauls from Britain. It is difficult to evaluate this claim since we have no Gallic sources and Caesar is clearly motivated to find a justification for his invasion of Britain. Remember that Caesar is writing for a Roman audience. He wants to simultaneously boast about and justify his military conquests. His claim is not implausible though, and even if such aid was limited in scope it is abundantly clear that there was much contact between the Gallic communities on both sides of the channel.

 

As for your second question, where he would land, this is much less obvious. Caesar himself did not yet know and his Gallic allies weren't helpful either:

yet he thought it would be of great service to him if he only entered the island, and saw into the character of the people, and got knowledge of their localities, harbors, and landing-places, all which were for the most part unknown to the Gauls.

^De ^Bello ^Gallico, ^IV, ^20

So Caesar sends one of his lieutenants to survey the coast while Caesar himself prepares for the actual invasion:

He sends before him Caius Volusenus with a ship of war, to acquire a knowledge of these particulars before he in person should make a descent into the island [...]

Volusenus, having viewed the localities as far as means could be afforded one who dared not leave his ship and trust himself to barbarians, returns to Caesar on the fifth day, and reports what he had there observed.

^De ^Bello ^Gallico, ^IV, ^21

When he does arrive at the British shore he does indeed find an enemy army awaiting him. Even after relocating to a more favourable landing site the landing is contested, but ultimately successful.