Unlike many countries, the UK doesn’t have tipping as part of normal life. While our US visitors will be used to routinely tipping 20%, the norm here when tips are given is more like 10% and even then it’s usually for excellent service rather than just expected.
In self service places and bars, tips are not expected. If the bar man has been particularly helpful and you are feeling generous, then ‘have one yourself’ is the phrase used and the bar person will serve themselves a small drink or take cash to an equivalent amount.
A tip of around 10% for good service in a restaurant would be normal but a lot of the time people just round up – to the nearest £1 for coffee/cake and perhaps to the nearest £5 for a meal. Tips are often pooled between he front of house and back of house staff. Some restaurants will automatically add on a service charge of 10-15% : if they do this then tips on top are not normal (but some very cheeky places ask for tips too). I always think it’s a very poor : I expect the cost of service to be built into the prices I’m changed.
However, be aware that those in the tourist and hospitality industries are often poorly paid seasonal workers. About half will be on the minimum wage (£6.70/hour for over 21s, rising to £7.20/hour for those over 25 in April 2016). Your tip of 50p or £1 adds significantly to their income and will be very much appreciated. Tips can’t legally be used to top up low wages to the legal minimums.
If you are on a coach tour, then at the end of the tour, a small gratuity to the driver and guide would be usual : the amount very much would depend on how helpful the driver or guide was: above simply doing their job.
Taxi drivers are normally owner-drivers and with taxi licenses strictly regulated, most in cities earn a good living so rounding up to the nearest £1 is about as much as anyone goes – unless they’ve been particularly helpful with luggage etc. If they sit in their seat and leave you to wrestle with doors, luggage, etc. I’d give them not a penny extra.