I usually Photobox for my printing needs, but I got the canvases hanging on my wall from
https://www.canvasdesign.co.uk/ a few years back. I agree with Cras in that these services are all pretty similar.
Just don't get over excited when they are delivered and repeat my mistake of starting to slash away at the packaging with a penknife before swiftly realising that that's not really a good idea. Nothing was harmed, thankfully!