I think by location the OP means British. In reality there isn't such a company, Arnold Clark is always recommended by they are only really Scotland and Northern England. Otherwise local means one town or county where one-way rentals would be pointless. Everything else is the usual internationals and franchises thereof - and some of these are only in the airports or very large towns.
Enterprise have fairly good coverage of locations, in Cornwall they have one at St. Austell less than one mile from the railway station. But this idea is a bit odd, why pay to rent the car that day then leave it and pay train fares all the way to London? It would make more sense to find a drop off location in a west London suburb, near the M4 and a station, like Brentford where Enterprise is less than half a mile to the station.
Or does London in the question really mean a London airport?