Trek 1.1 and Specialized Allez have both got good reviews in cycle magazines.
Triban 3 is £200 cheaper, but not £200 less of a bike. It has all the same key elements as the more expensive bikes, but with slightly cheaper finishing (tyres, chain).
Either of these options would be a good choice.
Also Boardman Comp is well worth it, but above your budget.
In the classic "look what you could have won" angle of a magazine review, upgrading to a Tiagra groupset (gears and selectors) gives you a much more rideable bike, but costs a few hundred more. If you have the money, have a look before you settle at the £500 budget.
But there again, add a few more hundred on top of that, and you are in Carbon Frame territory. And it goes on from there.