I'd say: don't underestimate how crowded and tedious busier motorways can be!
Personally, I'd get the train from London up to Scotland, and do a motorhome tour there. Then fly (to Heathrow/Bristol/Southampton) or get the train back down, and use a car (or motorhome, depending on number of days) to tour Wiltshire. Depending on what you're doing there, you might enjoy a narrowboat on the Kennet & Avon.
I wouldn't drive across the channel - the steering wheel and everything will be on the wrong side and the repatriation costs of the car/motorhome would be astronomical. If you want to get the ferry rather than the train, I'd recommend taking one of the longer ferry routes like Harwich - Hoek van Holland or Portsmouth - St Malo - doing Dover-Calais as a foot passenger is pretty tedious (with the notable exception of the view of the white cliffs on departure).
Once in continental Europe, make sure you don't underestimate how busy roads can be again, particularly if you're around school holiday time. If you can plan a route that is primarily on back roads, with more frequent stops, rather than a long run from, say, Amsterdam to Milan, I think you'll save yourself a lot of frustration.
If you're planning on driving through Switzerland, do read up on the motorway vignette.
If you're planning on doing a one-way trip, then you'll likely need to leave it in the same country. Perhaps you could consider leaving the motorhome somewhere in France somewhere near this Swiss border, and getting the train to Zurich airport?