Possibilities for free or low fee camping in Brtain seem to be increasing in some area but in others car parks and parking areas are banning motorhomes so it can pay to do research ahead of time.
When we first started motorhome travelling in UK in 1997 we had no Internet and little access to information so had to make it up as we went along.
If we were ever visiting a pub or restaurant we would ask the owner if we were could stay overnight in the car park and were always told yes.
Many small towns have car parks where the local council will permit you to stay if you ask.
We often write to people in the hospitality clubs to find out whether they have a place for us to park overnight. Couchsurfing actually has a group for people who either have space for a motorhome to park or need a place. We had an unexpected surprise after parking at Kevin & Ruths. Inspired by us they sold up their home and set out full time travelling in a motorhome.
It is often worth joining one of the clubs to have access to their sites or information.
One of the newest "clubs" you can join is Brit Stops . For just £25 a year you are issued with membership and a very explicit directory. This shows you where the stops are located, the type of stop (pub, farm shop, vineyard etc), facilities, info on access and more.
The Caravan Club has some great CL's - these are locations often on farms where they have space for up to 5 caravans or motorhomes to stay overnight. Fees are low and facilities often minimal.
The Camping and Caravan Club is much the same but also has people camping in tents.
The Motorhome Stopover lists over 400 stopover options.
UK Motorhomes is a very interesting site and on it they have a directory of stop overs
Wild camping is a site where members post information on good places they have found for wild camping.
Recently we were trying to arrange a meet up with friends in their motorhome and wanted a pub where there was overnight parking. A simple google search keying in the country + pub + motorhome gave us some excellent results although we did have to be detectives to narrow down exactly which pub it was in some cases.
Don't be afraid to look for other alternative parking spots but as a courtesy please consider other people in the area you are parking. Ask for permission if possible and behave responsibly and don't draw attention to yourselves in order to maintain these possibilities for others.
Apparently in Scotland you have a "right to roam" but must still observe local signs which can be a little sneaky as we saw some advertising "no parking between 1am and 3am".
Above all we always thing that if you show CONSIDERATION for others when choosing a parking spot you are much more likely to be left alone.