I’m arriving in London and plan to spend several days there before going to Bath followed by Wales.
Is it easier/better to pick up my car in London or take the train to Bath and pick up there?
Bath. Unless you really want to see the M4 motorway.
You might consider taking the bus directly to Bath from Heathrow as in the RS guidebook. The cab fare to the rental car agency is worth it, as is the opportunity to adjust to driving on the left in the quieter environs of Bath.
Another option is to rent at Oxford.
Room to park a car in central Bath is very cramped. Renting from there should be more practical.
Train from London (Paddington) to Bath. Advance book specific train for lowest price - about 10 weeks out.
See You Tube Video.
The disadvantage of renting in Bath is that the stock & choice is likely to be lower, particularly if you need an automatic at a busy time so you have to be luckier to get a good deal. Car hire prices & availability is starting to improve but isn't back to pre 2020 yet by any means.
Just another thought, we flew into Bristol on our way to Bath. We picked up our car at the Bristol Airport. A good variety of cars available. It is about 20 miles from Bath to Bristol Airport.
I rented a car at Heathrow after a short stay in London many, many years ago for a 10-day drive around England. Having to negotiate rush-hour traffic on the M25, right-hand drive, and a stick shift all at once was a nightmare. I would reserve a car in any good-sized city and take the train there it pick it up. We turned our car in at Cambridge on our way back to London for a few extra nights.
The Brooks Guesthouse in Bath provided parking options for guests.