Addison Lee are fine; they are the largest private car operator in London, and pretty reliable. Drivers are a mixed bunch, but if you need a quick pick-up anywhere, they are a good choice.
Annoyingly, they've stopped doing child booster seats, which means I've stopped using them. Also, the one time they provided a baby seat, it was filthy.
I now use Savile; drivers are very professional, they use S-Classes and 7 Series, and prices are not OTT.