A customer can voluntary give a tip to show gratitude.
If paying by cash, a tip gets separated from the rest of the cash (the one that covers invoice costs) and is not visible on the invoice or tracked in any way.
When paying by card (POS) a customer can pay gratuity in addition to the basic price. A gratuity of 5 to 20 percent of the price is customary when good service is provided. A customer can accept the calculated percentage or enter the amount (in £) on the keypad of the POS terminal. Gratuity will be visible on the invoice.
Service charge is common for the service sector (e.g. restaurants). Service charge is optional and the customer can ask for modifying the amount or to totally discard it.
If the service charge is already added to the purchase, gratuity will not be offered to the customer on the POS, while paying.
Gratuities and service charges are tracked and visible on the invoice. You may find them recorded in the invoice reports.
None of these tips are taxable.