We had good experience with it in Washington D.C. and might try it again when we visit London in June. Specifically, to get from Heathrow to our hotel on the south bank of the Thames. Thanks.
Haven't used Uber outside of the states but I've used Green Tomato in London. They work exactly like Uber but are not quite as expensive. They operate in DC and London I believe and they are allowed to work Heathrow airport. We used them in and around London quite a bit and they were great!
Just got back from London a couple weeks ago, we used 2 car services while we were there and had great service and good prices with both. The trip back to the airport is always cheaper than the ride into the city.
Hummingbird Cars
T P Cars
Are tips customary with a car service?