Village hotel Filton is pretty good.
Free parking (up to 4 hours - no need to register your car)
no key card for the lift
the hotel itself has a large non-residents gym and starbucks, so there are always a lot of non-residents coming and going in the lobby, meaning the reception staff don't look twice at you.
(I sometimes think quiet hotels are bad, because the reception likley knows they have WGs there, and seeing a man walk in every hour or two, then walk back out, that isn't a guest must be really obvious)