In reply to Andy Johnson:
I've used a few, mainly to do big wall prints of various types so I look around driven by price and quality and available size options depending on what its for and where its going
Bonusprint - Got some really good acrylics done at reasonable prices, however most recent print appears to have been adjusted/under inked/printer is not calibrated well. However they have accepted it's not right and are sending a replacement at no cost (and I can keep the pretty good but not quite dark-enough one) - Was wary of them at first due to price, but pretty satisfied.
Photobox - I love their very basic (and quite cheap) Mounted Poster prints... they make for quite nice wall art just stuck on with 3m foam tape. Again though they have been a bit hit an miss, with 1/4 orders being apparently under printed or lighter than intended. Again though I can't fault the customer service. entire print was redone at no cost, no need to return (again useable but not perfect prints). Their high quality standard posters so far have been excellent, and massive and still affordable. very pleased with recent 150x100 cm Prints of a Swan (challenging blacks and whites), and another very colourful Kingfisher - but dont buy full price... wait a few days and there'll be an offer.
www.Whitewall.co.uk - Seem to be a much higher quality outfit, and their prices reflect that. Much wider range of paper options (both mounted behind acrylic and on other substrates). I've only had one big poster done by them on their Ultra HD paper, which I can't fault, but I'm unsure if the difference is worth the price. They do have some very tempting fine art papers etc.