You'd think there wouldn't be much difference as alot of firearms are manufactured in both Europe and the US, so the prices should be roughly the same. However I think the British importers and dealers take the mick somewhat, due to the fact that it is a niche market and most go to their local gunshop - there isn't much choice in where to shop.
The US market is huge, everything is mass produced so their profits from individual sales can afford to be lower, also the amount of aftermarket accessories means most buy more than just a gun, scope and mounts. Just my tuppence worth...