I've always found it depends where you are. In the city centre it will be dead easy. Just look for the ones that have the Taxi sign on the top of the car lit up. If the light is off it means they are not available.
You are never far (in Central London) from any main street that a cab will go down - I wouldn't worry too much. And some black cabs have radio control, so stay away from mini cabs
outside the West End and City of London, and outside working hours, they can be quite hard to find. They're especially rare south of the river. That's why minicabs exist. But minicabs can be untrustworthy (or downright dangerous). They're usually just local businesses - if you're staying at a hotel, ask at reception. I've used this one a lot: