I am huge fan of London Black Cabs. If I am ever out in London at night on my own it is the only way I will travel home. But if you have a planned journey then Addison Lee is the ideal choice. An internet site makes it really easy to book a car in advance - and the experience is fast and efficient. The prices are about on par with a black cab but the cars are all a high standard, and whilst the drivers may not have the same 'knowledge' as London black cabbies, they do all have GPS instead.