I may be a little late to the game, I'm not sure, but Hailo is a bit to a recent revelation to me. In short, its a very simple, well designed free app through which you can book black cabs wherever you are in London. You either tap in your address or it'll use GPS to locate you, tell you how far away a cab is, book the cab where you can offer to pay in cash or card and the jobs done!
Ahead of the registered back cab arriving you will be given the driver's number plate and photo which is a really nice touch, especially for a feeling of safety if you're stranded somewhere late at night. I also love the fact you can register a card to your account, so if you're stranded with no cash to hand a taxi is no longer out the question — the fare is auto deducted from your card and a receipt emailed to you.
From the few times I've used it Hailo is an very smooth and hiccup free process — a powerful tool to have in your pocket.