Was shown this site buy a work colleague and it's great! It's all user submitted designs but they're all so unique! You won't find these designs in most shops.
How it works:
2 designs that have been highly voted will appear as the days deal where you can purchase them for £8 (take account for p&p not sure how much it is! Sorry!) but they are only there for 24hours before it gets put up to £10 on the second day as a last chance buy.
I'm yet to stumble on a design I like, but check it out as there are new designs daily and they occasionally do random surprise sales where you pay £6 and get a Random T-Shirt.