1.Normal Car: Fully tuned LCC Rocket' 07.
2.Super GT and Touring Car events: Fully tuned Honda HSV-010 GT 15th Anniversary' 11
OPTION 1: If you don't have the 15th Anniversary edition cars, and you are pretty rich, get the McLaren F1 GT-R' 97
Just a warning. The F1 GTR at Silverstone drives really ahead of the pack and pits off really late. And, once you pit, you're going to have a bad time with the 1st-placer F1 GTR.
The Rocket is an easy win for the road car class, due to the fact that with correct tyres, you do not have to pit at all in the entire race, making it a good choice.