We used AutoEurope last year. I was quite pleased. Best option for picking up at Pancras and they were really easy to return to at Heathrow (T5). I have also used Sixt before (used to be Sixt Kenning)
Some thoughts, along the lines of what others have already wrote:
- make sure you get an automatic. I am very comfortable driving stick but I find as I am getting older the novelty has worn off. Plus, you are shifting with the wrong (left) hand, which is annoying. Also, I learned to do hill starts with using the handbrake. Most rentals come with electric handbrakes now. Useless for hill starts.
- diesels are more economical and just as peppy (even more so at low range). A lot of rentals are diesel now.
- plus 1 on the small/narrow car consideration. Don't let them talk you into an SUV or a boatsized car. When you are trying to squeeze down a B road between a stone wall and a tractor, you'll be glad you're not in a Range Rover.
- if you are not comfortable on roundabouts, read up on the rules beforehand. Basically, outside lane(s) for the first exits, inside lane(s) for exits after halfway or further. If you come to a double roundabout (figure 8), lay on the horn, wave out the windows and just go for it (kidding).
- GPS is standard and mandatory. These people have no idea what a grid pattern is.. lol
Otherwise, enjoy !