In the UK, tipping in restaurants is usually 'discretionary', but it is more discretionary in some places than others. Check your bill. 'Service not included' means just that, and it is usual to offer 10 per cent.
If you are paying by card, you will often be able to add the tip before entering your PIN number. This is fine, but leaving a cash tip is more likely to circumvent the odd unscrupulous owner and reach the waiting staff.
Some establishments will add a discretionary percentage automatically. You are not obliged to pay this if service has been noticeably poor, and it is perfectly acceptable to ask for it to be removed.
Tipping is also commonplace in hair and beauty salons, and in taxis. Use your discretion, but err on the side of generosity.
In smarter hotels, tipping will be expected. Give a small gratuity (i.e. one or two pounds) to bellboys or porters per piece of luggage if they take your bags to your room. Doormen should be tipped upon checking out if they have helped with taxis or luggage. A banknote may be left in your room for housekeeping. Check whether a service charge is included in your room service bill. If not, add ten per cent at the end of your stay and ask that it be given to the appropriate staff members.
In pubs, don't tip - just offer to buy the barman a drink instead. In bars, where you are seated and drinks are delivered to your table, tip 10 per cent, as you would in restaurants.
Don't use tipping as an excuse to offload a pocketful of coppers and small change.