I followed your 4400 mile US cross-country trip in 2014 and thoroughly enjoyed the regular vlog. I felt inspired to do something similar within the UK, but that plan was scuppered when the young'uns came along shortly after.
When I looked into the viability of outdoors guerilla charging within the UK, it quickly became apparent that available sockets are a lot less prevalent than in the US. I suspect it have something to do with the strict EU regulations we have to conform to – It always amazed me that there are so many unprotected (non-waterproofed) outdoors sockets in the US.
The prior suggestion of using Google street view to scout a location is a good one, although this would obviously limit the spontaneity of opportunity charging. You'd generally be looking out for grey waterproofed wall-mounted enclosures:
Although not ideal, maybe using EV charge points is the answer for a reliable power source. The following adapter would let you use a regular mains charger with common type 2 EV charging points.
Campsites with mains hookups should provide a charging opportunity if you're looking for somewhere to pitch a tent. The following adapter would be required to do so:
Our petrol stations tend not to have regular mains sockets outside.
There's a large chain of supermarkets called 'Tesco' and their larger 'Tesco Extra' stores often have kids ride-on toys outside. A cheeky charge can sometimes be nabbed from the sockets powering them.
A 10m mains extension lead would probably be a wise investment so that you could use indoors sockets to charge outdoors.
If you plan to ride through Westbury, you'd be welcome to charge at my house or even sleep on the couch if you needed somewhere for the night.