Cardiff (October) is a flat one, Bacchus (September) (Dorking, Surrey) NOT a flat one, but a lot of fun. Gosport (November) is also very flat. If you want to go overseas Berlin (April) and Cologne (October) are flat. The Isle of Wight one (August) could also be a good one but I don't think it's overly flat. Other people have mentioned Bristol and Chippenham which have good reputations. Not sure about the Great Scottish Run in September (Glasgow) - I did the ladies 10k there and it was fairly flat but I don't know about the half marathon course.
Wokingham is on 10 February and pretty flat apart from two motorway bridges which are fine on the way out but horrible on the way back! Fleet is a gently undulating course and definite PB potential - 17 March. I'm doing both of them.
