I use them quite regularly as it is easier for me to get a car to SW1 versus HEX/Tube and cab. They quite often send larger cars than you ask for although I would not count on it for ensuring you have enough luggage. Salon (their smallest) is usually and Audi A6/Volvo S60 or Mercedes C/E class. I quite often get BMW 7 Series or Mercedes S Class but that could also be because of my usage. Prices start at £42 to central London post codes.
A bit of a poor booking system if you want to pay by card but they will call you up to take details.
They come recommended.