I’m not an expert, but from what I can find, there was a sort of “mysticism” associated with the ancient Britons and their swamps. They might have used them to stay cool in hot weather, as a ritual location, and as barriers to invaders who weren’t acclimated to traveling in them.

A lot of these claims seem to come back to these writings from Cassius Dio, and we know the Romans frequently described “lesser” societies in a very unflattering way, so it may be exaggerated.