10% is normal here in London.
It is polite to tip 10-15% of the taxi fare for black cabs and licensed minicabs in London. However, most people simply round up the fare to the nearest £1 and tell the driver to "keep the change". If you've had a longer journey and the driver has assisted you with luggage, you may wish to tip a little more, up to £5.
It is not a requirement to tip in taxis, but it is customary to round up to the nearest pound on metered taxi journeys, more as a convenience to both passenger and driver than as a tip.
On an airport journey in a booked minicab you might wish to tip two or three pounds if the driver helps with your luggage. If taking a licenced London taxi cab to or from Heathrow or in London a 10% tip is the average amount, although as in the previous paragraph, it is not a requirement, nor should it be expected by the driver.
There is a big difference between a taxi (usually a black cab) and a mini-cab. Anyone who can drive can become a mini-cab driver whereas taxi drivers have received a lengthy training, tend to know their way and serve better. They are required to take the shortest route between points. However, the price of a black cab is usually markedly steeper than that of other services, and some will refuse to tip on these grounds.
About 10% of the total fare is usual for licensed, metered taxis. Rural taxis and minicabs usually charge a pre-agreed, flat fare and many people do not add an additional tip.
Taxi drivers in London expect a tip of 10 to 15 per cent
The custom is to tip black cabs but not minicabs
http://www.independent.co.uk/news/uk/to-tip-or-not-to-tip-what-is-the-answer-1526498.html (dating from 1995)
What proportion of people leave a tip?
Business customers are more likely to always tip (7 in 10), while
around half of personal customers do
Taxi & Private Hire card payment, Transport for London 2009, p.57