Ask HN: print shops with APIs (UK-based)

Hi, I'm looking for a decent web-to-print API for a personal project where users will be printing business-card/playing-card-sized cards via a webapp. (eg, webapp generates PDF, punts to print shop's API, print shop sends real things to the user)

I'm a big fan of MOO, and I know it has a decent API, but the print costs might make it a no-go for my end users (they won't necessarily be happy to pay for the MOO paper stock quality etc).

Is there anyone else, ideally based in the UK (or which can reasonably cheaply ship to the UK) that I should consider?

Cheers, S