I've been very happy with prints from Costco here in the US. They offer profiled output at very affordable prices. $4 USD for 12x18 inch, $1.50 USD for 8x10
Dry Creek Photo offers custom profiles for virtually all the Costco machine printers in the US (as well as many other stores), but it looks like most of the Costco UK profiles are out of date.
Some setup like this might be more affordable than your pro lab if you can find someone locally who makes their output profile available. Heck, if you convert to sRGB like Farnells wants you to, you should get decent output even from a drug store printer, if you can get them to turn off auto-adjustment.
I looked up the prices at your pro lab (Farnells), and it's cheaper than one of the pro labs around here (Pro Photo Supply, Portland, OR). $18 USD for a 12x18, vs. GBP 8 at Farnells.
I suppose the bigger question is, do you want inkjet prints, or do you want RA-4 prints?
--Greg