Your contract of sale is with the retailer, if there is a warranty issue and the manufacturer has authorised that retailer to act on their behalf then yes. If not the retailer will have to operate under the conditions of the warranty as stated by the manufacturer. If that warranty states that the warranty is void if anyone not authorised by them to work on their products opens the guns up then they are supposed to send it back.
True, but that's the retailer's contract with the manufacturer, which isn't relevant to the consumer's contract with the retailer, and our statutory rights regarding fitness for purpose, freedom from defects, and durability. Those lie with the retailer - any manufacturer support is a bonus.
I'd be astonished if any UK retailer is
actually sending broken toys back to the manufacturer.
Maybe at the TaiwanGun level, but even PatrolBase just punts them on as boneyard guns.
Anyway, to answer the original question: I can't think of many. Kingdom of Airsoft will sell you a new gun from a very limited range
and a service for it, or a by-the-hour rate of £45. I wouldn't pay that much for someone else to work on a toy, but then I'd expect paying
less than that to create more problems than solutions.
PatrolBase don't list services, but they do a lot of tech work, so you could always ask. Again though, what quality are you really going to get from an assembly line?