Unless you particularly want to buy one separately, you might find the one which comes with the broadband ISP you choose, is perfectly adequate. Have a look at
www.pipex.net. They do a good start up package, where the modem is free, and there are no set up costs. They currently do 1Mb packages, for a little more than BT charge, but you haven't got the additional set up costs, and the service is uncapped, unlike BT.