I would agree with Lord B's recommendation for the Caravan Club's Tewkesbury site. We have used it a few times in the past when travelling from Central Scotland to the West Country. I found that getting the biggest "half" of the journey done on the first day, getting south of Birmingham,before stopping ,gave the impression of an easier second leg.
It has been some time since we last stopped there and I'm fairly sure that I read somewhere that due to problems with local youths a security gate was in place at main entrance/exit which limited early departures. Don't know how accurate that info is or whether it is still the same but otherewise a very handily located site, only about 10 minutes drive from the motorway, and a few minutes walking distance into the town centre.
Robert