Is it normal/expected to tip taxi drivers in the UK?

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.

http://www.visitlondon.com/traveller-information/essential-information/money/tipping

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.

https://www.tripadvisor.co.uk/Travel-g186216-s606/United-Kingdom:Tipping.And.Etiquette.html

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.

https://www.theguardian.com/money/2014/sep/29/how-much-should-i-tip-etiquette-tipping-service-charges

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

No it isn’t! I am English, have lived in Oxford and London all my life, have taken hundreds of cabs and generally don’t tip. I have never had any sign that tipping is expected. I occasionally tip, either if I really bonded with the driver, enjoying a good conversation on the way, or if I was late and the driver waited for me. Otherwise, no. Some people tip, some people don’t, but there is no convention, and I would suggest that you don’t do it, unless you receive exemplary service.

I’m afraid my answer will be nothing more than anecdotal. My source is that I lived in London for a while. The bottom line is that no tip is required. However some people do tip, as it often happens in these cases, be it for habit, or because they feel they have to.

Minicabs

With minicabs (in London) I always agreed the fare in advance, and made sure the driver did not attempt to squeeze a few extra pounds from me by quoting him the price before paying. Anything as simple as “It’s £15 right?” will do the trick. I usually never tipped, unless the driver provided an extra useful service, such as helping me with heavy luggage for example. This last consideration applies to minicabs outside of London too. I also tipped one particular driver, who picked me up so many times from Heathrow that I used to ask for him specifically when booking.

Black Cabs

As far as I remember I never tipped a black cab. I certainly never tipped 10%. I always thought of the cost of the service being so high that no tip was needed. I never heard drivers complain about this. Maybe I was (am) just cheap. Or maybe it’s just the way I am used. Moreover, now that card payments are widely used, I don’t even know if there is an option to add tip before paying.

It’s not a requirement to tip taxi drivers in the UK. However, it’s generally accepted as the ‘done thing’. Note however there are different taxis – the famous ‘black cabs’, that have official rates, and other unlicensed minicabs where you’ll need to organise a rate before you start driving.

Source – used to live in London, and tripadvisor.

Yes it is conventional to tip taxis in the UK, just rounding up the fare something like 10%-15% is more than adequate. A professional licensed cab driver isn’t going to say anything if you choose not to tip but unlicensed minicabs might ask for a tip. If you’re getting a minicab from an office (they often have a yellow light at night) then agree the fare in advance.