HM Customs and Revenue assumes that cab drivers get an average of 10% over their fares in tips, at least in London. Most restaurants, especially the cheaper ones, do not include a service charge but many do. Don't tip directly with money in a bar/pub. The traditional way if you want to do so is to finish ordering a round with 'and one for yourself', and he/she will take the cost of a drink (but not necessarily the drink itself). However, this is done more by regulars so don't feel that there is any obligation to do this, and certainly not each for each drink you order!