If you are in a pub and only get one drink you don't need to tip. Infact I would say most people don't tip in pubs. At a lot of restaurants in London a 12.5% service charge is added to your bill so you wouldn't need to leave anything else but there are also a lot of restaurants that still don't add a service charge. i.e one of my favourite restaurants in London is a chain called Cote Brasserie. They are very reasonable but also very popular. They add 12.5% but I always leave extra as the service and food are always first class. Infact I am off there tonight and was also there on Friday.