A good halfway approx stop would be Tewksbury Abbey Caravan club site. No more than 5 minutes from M5.
Further down the motorway,longer first drag from Doncaster but shorter next day to Newquay, would be the Caravan Club site at Tiverton. Again no more than a couple of minutes from the M5.
Paul, I am from near Grimsby and last year on the way down to cornwall we stopped at Hurn Lane Caravan Club Site, Burnham-On-Sea it was just over half of the journey and is not far from M5