I would expect to pay a heap of duty if importing from outside the EU
First of all, if you want anything in the UK you are going to be buying online. If you are desperate for something simple like a new mouse then there are retail stores PC World and Maplin, but they really aren't worth looking at for anything more complex. Think of Best Buy but not as good.
Some of the well known UK sites - I'll keep to those I've used regularly for now. Others will probably argue with my comments, everyone will have favourites.
http://www.scan.co.uk - has possibly the widest range of enthusiast class kit. For example I seem to recall they had the widest range of solid state drives when they were just starting to appear a few years back. They have a daily page of today-only deals.
http://www.overclockers.co.uk - another enthusiast specialist, e.g. lots of watercooling stuff, deals change weekly.
http://www.aria.co.uk - similar again. They have some pretty good very limited deals that update every day - typically 20 or so of each item at much reduced prices. Their range is a little smaller than those above, I think.
http://www.novatech.co.uk/ - a bit smaller range than the above. More known for their custom systems. I've only really used them because they have a real shop I've occasionally visited.
http://www.ebuyer.com - are a little bit more "pile it high, sell it cheap" than the above. They ship for next day delivery until 11pm which is nice, although in my experience that isn't guaranteed. More businiess oriented stuff - wider ranges of things like printers than the enthusiast sites.