Viz does sell manga in the U.K>vizeurope.com>>12248410
There are a number of shops that import but finding them can be a pain. Funnily enough the Waterstones near me sells Reborn! + Claymore volumes 5 - 10 and they will happily order volume 11 or the prior volumes. Also, they're currently running a 3 for 2 deal on all Graphic novels. (I actually just picked up the Watchmen today)
If you can get to one, Forbidden Planet is really the place to go.