Do not simply go for the cheapest - big mistake - anything under about £22 is unlikely to be the best around, simply because there isn't enough margin for profit (or system maintenance) after paying BT. There are certain ISp's which have attracted a lot of negative comments recently (tiscali springs to mind, as do V21). Problems have included low speeds, no connection, billing problems.....is it worth risking for the sake of a couple of £ a month?
From what I gather, Pipex is amongst the best for a balance of price and quality at the moment - it's £23.44/month with free activation (you pay a fee if you leave in the first 12 months).
The comments on this board have been generally good from Pipex users - I believe there have been periodic problems (but hey, no company is perfect), but they have been resolved pretty quickly. I am not with them.
By the way - I am assuming you want uncapped 512, as there are capped services that are obviously cheaper.
You can also get slower speeds (256, 150) for less money as well.
http://www.solo.pipex.net/
I am with Plus Net, and I can happily recommend them from a service point of view, but they charge the activation fee up front, and charge £22/month thereafter.
I hope this helps.