The practice of tipping varies from country to country according to legislation and the prevailing wage structure. Before travelling, it is worth consulting a guide book to ensure that you are familiar with the custom and do not cause offence.
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 at least ten 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 proprietor 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 in some circumstances it is acceptable to ask for it to be removed.
Tipping is also commonplace in hair and beauty salons and in hotels. Use discretion, but err on the side of generosity.