Many restaurants now add a 'discretionary'
service charge to your bill, but in places that
don't you are expected to leave a 10% to
15% tip unless the service was unsatisfactory.
Waiting staff are often paid derisory
wages on the assumption that the money
will be supplemented by tips. You never tip to have your pint pulled in a pub.

If you take a boat trip on the Thames you'll
find some guides and/or drivers importuning
for a tip in return for their commentary.
Whether you pay is up to you. You can tip taxi drivers up to 10% but most people
round up to the nearest pound.