So there's an old theory that Scandinavia's an awful place to live but it somehow got overpopulated, so Scandinavians began to go a-viking so they could find farms to feed them and brides to breed them. Because of the way that copyrights, Google Books, Wikipedia, and the interweb work, the average person looking for historical information is much more likely to encounter one of these hundred-year old theories than anything that resembles the current state of scholarship. This helps to explain two assumptions in your question: (1) Norway was rough! and (2) Britain was great!

We actually know that Scandinavia had a lot of room to grow during the Viking Age. There's general markers of population increase, urbanization, intensified agriculture, the expansion of land under cultivation, a more diversified exploitation of resources, increasing ties between regions, and more interdependence among communities. Historians and archaeologists can see evidence for these things across much of northern Europe between 800 and 1100, although certainly there was some regional variation and some ups and downs across time. But the general picture is that northern Europe—including both Norway and England—were producing greater and greater surpluses during this period.

On to England. The Roman economy had depended heavily on government contracting. A senator would receive the government contract to supply a legion outpost, and then he would use the government convoys to transport his own goods as well—subsidized trade. When government tax and supply convoys ceased in northern Europe, this trade economy completely collapsed pretty much everywhere north of the Loire. Within the space of a generation, basic goods like iron nails seem to have disappeared completely. The 500s were tough, and the 600s were maybe even worse. Changes in the environment, shipping technology, the organization of governments, and methods of gathering agricultural resources began to turn things around in the 700s. King were starting to feel good about themselves and cobble together decent-sized kingdoms, although their armies probably never numbered more than a few hundred troops.

So from this perspective, it's pretty easy to see where the early viking attacks came from. Everyone was increasing in wealth, but the increase was uneven, so some people decided to steal from their well-to-do neighbors. But among the well-to-do of relatively prosperous England, all they could ever hope to effectively manage was a few hundred people, and that would take time and pulling strings. So there was plenty of incentive to go off a-pirating, and a few boats of pirates could really shake things up, and that's exactly what happened.

As economic ties between the North Sea and south Baltic started to cut Norway out, some people from Norway decided to leach some of the surpluses out of England. This raiding posse probably represented the muster of a single fjord, but for the petty kings of England, that was a huge threat. For England as a whole, though, it wasn't that big of a deal, and few people from Norway probably even realized that their neighbors had spent the summer touring sunny Northumbria. Certainly there's no Norse myths about the earliest raids, which suggests that the Scandinavians didn't think they were that big of a deal.

But this was all in the 790s. I think there were major changes in the 830s, when the Norse started to occupy home-bases in Western Europe (and when many Western Europeans began to adopt the Norse practices and identities of their successful immigrant neighbors). And again in the 860s, when these hybrid communities began to spinoff permanent standing armies that were political communities in their own right. And things kept on changing into the 900s and 1000s, although my knowledge of these periods isn't as precise. If you took a Norse army from one of these later periods in the Viking Age, it might look a bit like ISIS building destroyers to attack the US. But if you think about what the situation was just in the 790s, the earliest raids were plausible and could perhaps even have been expected.