Tipping is much less common in the UK than it is in the US.
Generally, restaurants will add in an optional service charge of 12.5% to your bill. This is completely optional and you are welcome to leave more or less as you see fit. Taxi drivers do not expect a tip but are very grateful for anything you give them. Rounding up the fair for a typical ride or giving an additional pound or two if the driver offers extra services is greatly appreciated. Tipping at hair salons and spas is generally 10-20% depending on your service.