We hired several before we bought.I’d recommend hiring in France, or Germany. I certainly wouldn’t recommend seeing the uk via motorhome.
We found the best place was Germany, it's a lot cheaper than the UK and you get much better motorhomes for the money.
We used to load up the car with all that was needed (so we did not have to hire bedding, towels, cutlery, cooking stuff, deckchairs etc)
leave work on a Friday night, catch the overnight ferry from Harwich (40 miles from the M25)
Spend the night in a nice cabin.
Arrive in Holland in the morning and be at the hire places in Dusseldorf by lunch time to pick up the van and do the hand over.
Park the car on the van hire place and be on our first site an hour or two down the road by 5pm.
Personally, I'd book something up for next May to September.
You will have a lot more fun in Germany in the summer (in a nicer motorhome) than you will in the UK in the middle of winter.
(I'd guess it may be cheaper as well)