Don't go to halfords, go to an independant bike shop, Halfords artificially inflate the price of their bikes to more than double what they should be then sell them for half price to con you into thinking you are getting a good deal. A £100 bike will more than likely start to break after 100 miles or so (I had one and in less than a year almost every part needed replacing except the frame) and you don't want your cheap brake levers suddenly snapping when you are going fast down a hill. Pay a bit more and you can get a decent quality second hand bike on the net from a quality manufacturer.