Usually Wiggle with their £10 next day delivery for a year, and the platinum discount, and our evri delivery guy is frankly, brilliant. Never misses a delivery, never pisses us off. Their stock levels can be a bit hit and miss though.
Tredz seem to have some good deals, and importantly, lots of stock, so I’ve used them a fair bit too.
Otherwise Sigma (NHS discount helps), and Merlin.
My LBS is Wheelbase, who only stock stuff at full whack, and even though I’ve spent £15k on bikes with them in the last 4 years, they still wont even consider a discount…hence very few bits from them, unless I’m really desperate.