Car rental in and around London

How about getting a train out of London to you first stopover place (Bath, Oxford, Cardiff, ....) and pick up the car when you leave there. Avoids the mad dash down the motorway, you just get the more pleasant local drive at the end.

"close to an M or A" - not sure where you mean.

Slough would be a good place to pick up and/or returning a car. Slough is 7 miles from Heathrow, a 15 minute cab ride.

Slough, or Reading which you can reach by bus from Heathrow.

Near an M or A - near one of the freeways or highway with an M or A designation.

So Slough would be a great place to drop off a car on the way back to Heathrow.

kbharris, sorry for my dumbness.
The roads with numbers starting with 'M' are called Motorways, M1, M2, M25 etc. The are purpose built roads for motor vehicles only, same as a German Autobahn, French Autoroute etc.
A roads are the next level down, and are normal roads (all traffic allowed, pedestrians, bicycles etc.). They can be fast roads outside town, single or double carriageway, or in towns they can just be a normal street. There are thousands of them, it is difficult not be be near one.
"Freeway" is not a term used in the UK, and "Highway" is a somewhat old-fashioned word, it can just mean any main road.

Yes, Slough is a famously ugly industrial town just west of London, but it is near enough to Heathrow (under the flight path) that you can get a taxi from there to Heathrow. Or just drop off the car at Heathrow, which will probably involve a shuttle bus from an outlying car hire compound..

Sorry for the Americanism. I couldn't remember the terms. Having driven in England in the past, I know that if I can spend the first hour or so driving on one of the larger, multi-lane roads, it gives the brain a chance to go "oh, ok I remember this". So being able to get quickly and easily to a motorway is another thing I'm looking for in a rental location.