Rich in what context? They controlled a comparatively large amount of land; they would have money solely based on the size of the population and taxation. However, smaller kingdoms were producing as much wealth with fewer people, and had more developed lands. The Normans (for example) were producing far more, despite having a smaller territory (and generally less rich).
And they were pushed around, depending on era, by the Welsh (their deals with Mercia generally reamed Mercia), by the Picts (smashed the Angles up badly, and forced them to relinquish much of northern Northumbria, which almost immediately fell under Scottish control), and by the Scots (similar early circumstances, as well as Robert the Bruce's royal screwjob on Edward the II and III), and Danes (the Danelaw).
The Saxons were poor, considering the size of their population and holdings; they might have had more money than X kingdom, but they should have had MORE, and would have had with better management. It was such a problem that Alfred the Great resorted to burhs to defend his land; why would he do that if he had the money for a proper defense? He lacked the money to defend everything he had with a real military, and had to resort to using peasant-built and maintained burhs to do so. He couldn't even afford to pay those who built the burhs. Instead, he had to guarantee them plots in the burhs. If he had properly been exploiting the resources England provided (as the Normans did), he could've had proper forts instead; the Normans clearly did better with the land developmentally than the Saxons had.
Isn't to say Alfred was a bad king (clearly, he wasn't), but the Saxon governmental model ensures much of what went wrong economically, really wasn't his fault. So many mines and quarries were unused because of local powers that just didn't see it necessary; they were happy just getting by, and paying their tributes, and offering soldiers. In fact, utilizing their full local resources would probably attract unwanted attention in additional tributes being required, or invaders. Alfred couldn't be held to concern that a tin mine isn't being utilized; the Saxon model doesn't tend to address those things, and leaves more freedom of those decisions in local hands. The Norman model allows one to essentially force their lords to develop everything they can, and pay a larger scutage, increasing the royal treasury substantially more than one could get on the looser Saxon model.
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