There will be plenty of runners at both events and neither course is considered hilly. Even though Silverstone gets more runners, you wont be stuck for people at your speed at either event - No matter what standard you are.
If you are running 7 miles now, a half marathon by next march will be no problem for you. Personally I prefer Silverstone because i like running on the racetrack - But i've also had some great races at Milton Keynes, which is mainly on traffic free paths.
Its a really close call between the 2 races and for me it has always come down to other commitments in that month usually meaning i cant do 1 or the other.
Good luck!