I found Outline Editions by accident on my lunch break, on Berwick Street. It’s a great little gallery, full of affordable and very cool graphic design and Art. Most are limited editions and the prices are very reasonable, from around £30 to £200, so not only will they look great on your wall, they’re affordable too.
If you’re near Berwick Street, make a point to go in and chat to the staff (they even let Ted the hound in) and have a look at the latest exhibits
Here’s a link to their website, and some pics of ones I particularly like.