Okay, I'll give it a try since you haven't really gotten an answer for what you seem to want.
I'm going to operate under the assumption that a Charlemagne-esque conquered Britain, or England. Rather than having the english fortify their shores, I'm going to operate under the thought that a unified Britain/England would have looked outwards more and developed a navy for trading and potentially even allowing them to attack mainland Europe. This would in turn allow them to deter the Vikings, since the greatest advantage of the vikings was their ability to strike at will and then flee before their enemy could strike them back. That seems, to me, to be a more likely development to occur than them heavily fortifying their coast.
The first thing that needs to be considered is Britain vs England. You specified all of Britain, but later said England was fine. If it is just England, I could see several indecisive battles with England before the vikings would focus more heavily on the rest of the Britain Isles. This could in fact make the vikings more successful. If say the Great Heathen Army landed in the Edinburgh area and instead attacked Alba, there's a decent chance they could have overrun Scotland. The Highlands would have presented something of a problem, but I could see south Scotland falling. The Norse would have also focused more effort in Ireland, where they had established several trading posts like Dublin. This means Scotland, Ireland, and Wales would have been under greater assault than OTL simply due to being easier targets, like the Hebrides in OTL. Of course Normandy and Frisia would have probably similarly fallen under greater assault in France. However I am wondering if the English with a fleet might have either made common cause with the Franks to expel the vikings, or possibly even invaded viking conquered land right after to take it themselves. If the English were a sea power during this time, there's no way to predict just how much they would have interfered with the vikings.
If all of Britain was unified, then the vikings would either focus entirely on Normandy, Breton, and Frisia or simply abandoned the North Sea entirely. I'm actually leaning a bit more on the latter. If Britain had a navy then they would have dominated trade in the North Sea. So either the vikings would have been forced to fill smaller niches than OTL or maybe even the English would take the intrusion of another sea culture badly and driven them off. If the vikings couldn't trade or raid in the North Sea, they would have no choice but to look east to the Baltic Sea. The degree by which they focused east would just depend on the Britain vs England question. The lack of wealth to raid and steal would have made the vikings focus more on settlement and building up the Volga and Dneiper trade routes. This would mean a greater Varangian dominance over the Rus, making Keivan Rus more Nordic, more viking conflict with the east and west slavs, and probably a greater Norse population base around the Baltic. The Norse would also thus be more likely to be influenced by the Byzantines through the Dneiper and the muslim nations around the Caspian Sea than western Europe like OTL.