Thought I would give a very positive review for Europcar at Heathrow airport.
We picked up two rental cars from them (tip: two small rental cars much cheaper than hiring one big one!). Apart from guy at counter trying to sell an upgrade (which we were expecting) it was very easy and fast. Shuttle arrived within 5 mins of us getting to the stop, counter service was efficient and we were out of Europcar within about 15 mins, including choosing cars from the variety parked (slightly strange concept!) and getting the bumps and scratches noted on the forms. The car return was even better - we drove straight up, hopped out of cars, the guys checked them over, no problems, checked the fuel guage (full) and printed receipt for amount charged (identical to that on the booking). Then we queued for shuttle to airport, which took all of ten minutes. Two days later the NZD equiv amount was charged to the credit card.
So big thumbs up to Europcar....we'd def use them again.