Tipping is entirely discretionary but it is normal to tip at around 10% in restaurants in Britain (more if you feel that the service has been particularly good). Some restaurants have started to add a 10% or 12.5% service charge automatically to the bill. This is always discretionary so if you have been particularly unhappy with the service it is open to you to choose not to pay the service charge. Where a service charge has been added, you are not expected to leave an additional tip.
Pubs and bars
It is not expected that you will tip in a bar or pub if you are only buying drinks from the bar. You may want to leave a small tip if there is table service.
Tipping in cafes is discretionary and it is very unusual for it to be added to the bill. If you have received table service you may want to leave up to a 10% tip. It is not usual to tip staff when you have collected your own food or drinks from the counter.
If a porter takes your bag to your room in a hotel it is normal practice for you to give him a small tip. A couple of pounds would be more than enough. If you are in a small hotel or B&B and the proprietor helps you to your room with your bags, a tip would not be expected.
Tipping in taxis is not necessary but drivers appreciate it if you round the charge up to the nearest pound.