UK people get shafted on import fees these days (thanks government), dragging componants accross the pond adds another £50+ average. It's quite annoying to say the least.
I'd like to say the exchange rate on parts was fair, but it can be a bit random. Usually we end up forking out more for things; hence why I used to import practically everything prior to the import-rape-tax being added. Be weary if something doesn't have an obscenely high shipping price to here, it means your stuff will get caught in customs until you fork out for the import fees.
Amazon will happily import things without silly high import/shipping fees, but you're stuck on slow shipping for it & it can take months for them to arrive.
UK builders usually stick to a handful of companies for general part grabbing.
Aria,
SCAN (they actaully accept Bitcoin as payment now) &
Microdirect. The latter is great if you don't mind traveling or live in the Manchester area since they stock a lot that isn't on the site. London area people should just attack Tottenham Court Road, it's full of places with no online prescence & is pretty cheap over all.
There's loads of other places you could look, but the majority are flash in the pan companies that'll be gone as quickly as they've appeared. Best to stick to established places.