Unfortunately, the options for Britain are not that great. The default answer to this question in the US is to get the Kilo TT from Bike's Direct but they won't ship outside the states. If your budget could go up a little you could get a Dolan Kadet, which is probably a much nicer bike, but it doesn't come with brakes.

Converting a fixed geared bike to a singlespeed is relatively inexpensive if it already has brakes. You just need to buy a freewheel.

There are numerous threads on State bikes in this forum. The general opinion is that they are not quite worth the money, but most of the people who have actually ridden them haven't said they were completely worthless either. They mostly seem to be having an image problem, in that they are capitalizing on a trend, rather than being people who deeply care about bicycles.