musa, when i was looking a few moths ago, the general concensus of opinion was that £500 was the bench mark.
Models that have had good reviews at that sort of price include the Specialized Allez, Trek 1000, and the Giant OCR(?). There will be plenty of others too, and at this time of the year there should be some good deals knocking about because of the new models appearing, so you might pick up a bike with a better spec.
I think the most important thing to remember is to get the fit right, go to a couple of bike shops and get them to fit you for a bike. As a 'rough' guide measure your inside leg and then multiply be 0.65 ( the best way to do this is to stand with your feet 8 inches apart, put a book firmly between your crotch (of similar pressure you would expect from a saddle) and get someone to mark off the top point on the wall).
Have fun