My period is chiefly Anglo-Saxon, so I'll be able to answer more in relation to that, but if anything England (specifically Mercia and Wessex and later England c.800-1066) was actually fairly successful militarily, but has fallen victim to pop-cultural tendencies for Vikings, and to a certain extent its own propaganda.

There's something to be said in what you said here:

by England simply not being a united nation at the time

Many of the Danish victories in England come about as a result of English disunity. The 820s and 830s in England are marked by open conflict and a shifting in the balance of power between Mercia and Wessex. Although a fragile peace had grown into active co-operation by the 850s, the Danish force which attacks Canterbury and London in 853 fights the Mercian army before the West Saxons under Æthelwulf can arrive to reinforce them, and wins the victory. Similarly, the Danish invasion which takes Northumbria in 867 attacks at a time when that kingdom is riven by civil war and infighting. Even Svegn Forkbeard and Cnut's wars against Æthelred II in the late 10th and early 11th centuries are largely successful due to the infighting and political upheaval that spirals following the premature death of Edgar 'the Peaceful' in 975. Indeed, the Anglo-Saxon Chronicle actively complains that the previously-impressive English defensive network has been brought low by the inability of various nobles to cooperate.

Broadly speaking though, the Danish wars in England were nowhere near as one-sided as pop culture typically likes to depict, and if anything the English actually get the better side of things. As Asser states, the majority of problems that the English face in the ninth century stem not from battles themselves, but for the Danish propensity for avoiding battles. Raids were a key element of Danish strategy, and 'Vikings' were far more likely to run from anything approaching a fair fight than to try and fight one. Asser's complaint is that, when Danish activity is arguably at its peak in 871, Wessex is being 'bled white' by the massive manpower drain of constantly trying to chase down raiders and protect isolated communities while also fighting a 'conventional' war. If anything, it's impressive just how well Ælfred does, but the experience clearly informs his defensive reforms.

It should be noted that the English fair fairly well in battle against the Danes when they're able to force battle, or at least equally. While the Danes had victory over Brihtwulf's Mercians in 853, it should be remembered however, that in the following battle at Aclea, Æthelfulf's West Saxons inflicted "the greatest slaughter on a heathen army that we have ever heard of." In the same year, the men of Devon under an Ealdorman Ceorl win a great victory against a Danish force and "made a great slaughter of them there." In the preceding decade, the Anglo-Saxon Chronicle records eight major battles or Danish incursions:

840: The West Saxon Ealdorman Wuflheard of Hampshire wins a notable victory over a fleet of some 33 ships (likely some 1,600 men) attempting to storm Hamwic (Southampton). However in the same year, Ealdorman Æthelhelm of Dorset is narrowly defeated by a Danish army at Portland, and killed there.

841: The Danes win a major victory over Ealdorman Hereberht in Essex and then defeat an East-Anglian and Kentish army.

842: The Danes raid London or Canterbury

843: A Danish force of some 35 ships (c.1,750 men) defeats Æthelwulf at Carhampton

845: The men of Dorset get revenge for 840. Ealdorman Eanwulf with an army from Somerset and Ealdorman Osric with an army from Dorset engage the Danes at the mouth of the Parret (near Glastonbury/Minehead) and there "made a great slaughter and had the victory."

851: Ætheling Æthelstan (Æthelwulf's son ruling as King of Kent) and Ealdorman Ealhhere of Kent lead a fleet against the Danes at sea and win a great victory, capturing nine ships and putting the rest of their fleet to flight.

As you can see, the English generally put up a good fight, even if they are somewhat on the back foot. Their main handicap is that they are largely a defensive levy forced by circumstance to be primarily responsive to a highly mobile threat. When Danish activity reaches its arguable peak in 871, the English fight nine 'general engagements' against the Danes, explicitly not including the numerous "individual expeditions" made against raids and smaller incursions by forces under ealdormen and thegns.

In some cases, the West Saxon propaganda of Asser's Vita Ælfredi and the Anglo-Saxon Chronicle actively works against the image of the English, especially the non-Wessex polities. The ASC for example establishes the commonly-held view that Mercia was over-run entirely by the Danes in 873 and effectively ceased to exist until the 880s, but Welsh chronicles establish that the Mercian army under Ceolwulf II was actively campaigning and expanding Mercian influence and territory in Wales throughout the 870s and early 880s, and numismatic evidence suggests that Ælfred and Ceolwulf II were in fact close allies who saw each other as equals, rather than the 'foolish king's thegn' who was a Danish puppet recorded in the ASC. Indeed, it seems more likely that the Mercians fought the Danes to a settled peace in 873 rather than crumbling as recorded. A similar occurrence happens with the (in)famous Danish "victory" of 878:

While the Danes do surprise Ælfred at Chippenham in 878, it should be noted that this is because they take the extraordinary step of attacking in winter when most armies are stood down. Contrary to pop-cultural belief, however, their victory was an isolated one. A concurrent raid on Devon - apparently designed to take advantage of the chaos - finds the Exeter fyrd already mobilised. There, Ealdorman Odda effectively annihilates the Danes, purportedly killing some 840 from a force of around 1,150 men including Ubba, the brother of Ivar and Halfdene, and even capturing "the Raven", the banner beloved of neo-Pagans everywhere. That Ælfred's victorious army at Eddington the following spring when the campaign season resumes is drawn from the burghal garrisons of Somerset, Wiltshire and Hampshire suggests not only that those regions had not fallen to the Danes, but also that they felt sufficiently secure to dispatch the better part of their forces on campaign. Of course, for writers like Asser who were essentially compiling a hagiography, "brave Christian king overcomes insurmountable odds to liberate kingdom from evil heathens" sounds much more impressive than "defence-in-depth network functions largely as intended despite initial surprise."

The events of 878 clearly 'scare off' the Danes for a while, and the 880s pass without too much activity. In 883, an English force successfully defends London; in 885 the city of Rochester successfully defends itself from a Danish army until Ælfred comes to its rescue; and in 886 Ælfred and a joint West-Saxon and Mercian army successfully campaign against East Anglian Danes who had 'violated their peace' the previous year, culminating in the reconstruction and re-occupation of the Roman defences at London. Danish activity resumes in the early 890s, although aside from isolated victories against un-garrisoned fortifications and some raiding, the ASC records that "the army had not on the whole affected the English people very greatly at all".

By the 900s, the English are very much on the upper hand. While Edward the Elder of Wessex campaigned in East Anglia and later expanding control into the East Midlands, his sister Æthelflæd was consolidating control and building power in Mercia. In 911 her husband Æthelred won a significant victory over a Northumbrian and Danish army at Tettenhall, killing a number of senior Danish leaders. In 912, Edward campaigned successfully in Essex. In 913, a Danish force from Northampton and Leicester was eventually tracked by the garrison from Luton who "fought against them and reduced them to full flight and rescued all that they had captured and also a great part of their horses and their weapons."

In 914 a "great naval force" of Danes which had successfully raided Brittany and Wales was defeated by the garrisons of Gloucester and the burh at Hereford and 'the nearest burhs* (probably Worcester and Bath), and the earl Hroald was killed. Two nocturnal attempts to raid near Bristol - at Porlock and Watchett - were successfully driven off by local defences 'that so few could get away.' In the same year, Edward successfully captured much of Buckinghamshire and Northamptonshire and Bedfordshire.

In 916, the Mercians carried out a punitive campaign in Wales.

In 917, the garrisons at Towcester and Wigingamere successfully repulsed a surprise attack by a Danish force from Leicestershire, although they were able to capture 'a great many unprepared men and cattle' from the area. In retaliation, an English force captured Tempesford, and an army from Kent, Surrey and Essex captured Colchester from the Danes and made a great slaughter there. The garrison at Maldon also resisted an attack from an Anglo-Danish force reinforced by large numbers of Danish raiders, before combining with a relief force to put the enemy to flight and 'killed many hundreds of them' in a counterattack. The same year, Athelflad's Mercian army stormed Derby with great bloodshed, which was leveraged to force the surrender of Leicester and the submission of York the next year.

I won't talk about every battle of the ninth century but suffice to say that the English actually handle themselves fairly well, and come into the tenth century as by far the more unified, organised and aggressively successful military power.