hi, I dont usually respond to travel questions, as I always did my own thing which I found did not suit the majority, however in this instance I know exactly where the OP is coming from, done exactly the same thing on a few occasions, yorkshire to cornwall is a 12hr stint, set off to set up, the last thing one needs is hauling everything out, and setting up just for one night, maybe for only 6hrs or so, on a site that is close enough to the motorway, as not to mean a detour when all you want is to get where your going, to this end I have stopped at both Bridgewater, and Taunton Deane, both have motels and biggish carparks, park up, book in, something to eat, head down for a while and then up, shower, breakfast and back on the M5, as with all services the parking is expensive if you stay longer than 3hrs, but if you are staying in the motel it is free, take that off the cost of the room, and it's not too bad, considering you would be paying for a night on a site anyway.