You asked about UK rentals. If you want to pick up a car near Heathrow after a time in London, we found Sixt convenient. You can walk to the Hatton Cross tube station from the car lot. We had a SmartCar too!

When we looked last time, we didn't have the same concerns about pickup location and went by price and what was included. We booked a car from National through Auto Europe. They were fine.

Frances Toronto, Canada