I think it's agreed, even by Tissie and Borrie, that Dark Age Britain was fairly poor. But then again so was Dark Age Scandinavia so Viking labour was cheap. Cheapish. Dark Age Britain in the Viking Age was an organised political state (sometimes states) with an organised army and an organised navy so, at first sight, Viking raiding expeditions would have to have been mounted on a serious -- and therefore an expensive -- scale. [Obviously 'Danish' state invasions were but this is a different matter.]
It is impossible to envision 'booty' on the scale anywhere close to the costs of a large-scale, long-distance maritime expedition. Let's get real! OK, say you occupy and sack a small coastal town -- it would have to be small otherwise there would be a defensive infrastructure. What's on offer? Nothing in terms of moveable goods. This is a near cashless subsistence economy for Chrissake. What were you expecting? Let's hope it was market day so at least you'll be able to feed the troops. Actually the only thing on offer with any kind of cash value are the people themselves. So, OK, you bundle a few hundred bewildered peasants aboard and take them off somewhere. Hope you get past the Anglo-Saxon navy wherever you're going. Oh yes, and make sure you amend the Anglo-Saxon Chronicle before you go because there's no mention of such slaving expeditions. But the ASC does mention....