I went to Cornwall at the beginning of July travelling from Sheffield. I stopped at Michaelwood on the M5 south of Gloucester. You can park the car/caravan close to the services building. The facilities are also clean.
When we returned we stopped at Hopwood on the M42. it is quite a distance from the place where caravans park to the service building but this will not affect you going up the M5/M6.
Enjoy your holiday.
Steve