Bullseye Country Sports have an Amazon shop and the prices are adjusted for free postage - if you are a UKAPU member they will give you 5% discount so long as you email them first. I've found them very competitive for gearbox parts. There's also a bloke (whose name I totally forget, as is my usual MO) who does various TBB's at fair prices, but I mention him simply because he stocks barrels that are difficult to get anywhere else.
Something I find really annoying about shopping for airsoft bits on Amazon is getting to checkout only to be told that the seller doesn't ship to the UK V^^Vgrrrrrrrrrrrrrrr, why list them on amazon.co.uk then ffs!?!