The only people that it is customary to tip are those in the hotel and catering industry. Unlike the United States, bartenders, taxi drivers, hairdressers and guides do not expect to be tipped. This does not however mean you can’t! In the case of bartenders offering the bartender a drink is a good alternative.
The smallest note that you can get in the United Kingdom is £5 which is quite a lot of money, so when tipping hotel porters don't underestimate the value of £1 Coins.