A: Uk shippers charge a lot.
B: They don't get the subsidies that "poor" countries often do.
C: I think they virtually all use the UK version of eBays Global Shipping Program, and it's VERY expensive.
D: Probably other reasons too.
I once (grudgingly) paid $40+ shipping for something from the UK and it arrived at my home in Oregon *literally the next day* — before dinner, even. I'm still astonished to this day. It was sort of worth it, if only for the novelty. I surely didn't need a razor that fast
UK international post costs are relatively high, and they've gone up quite a lot in the last few years. They're broadly comparable to, probably actually a little less than, shipping stuff internationally from the US, which is pretty expensive compared to a lot of countries. And it's to do with reduced public expenditure for postal services.
The really big kicker though as others have said is ebay GSP, which is crazy.