Every time I've rented in the UK I've rented at Heathrow, returned to Heathrow, specified a manual (much cheaper) and paid for the first tank of gas so I don't have to prowl the neighbourhood looking for a petrol station. I've not returned inside London as I would prefer not to drive in the city. It's an easy ride from Kensington to LHR either on the Tube or Heathrow Express.
I've used Enterprise near Terminal 5 and booked the vehicle from North America. I've gotten excellent service from them.