My rule in UK restaurants is 10% tip - although I do give upto 15% for excellent service.

But.... beware of UK restaurants who add a "service charge" to the bill. I have no problem in paying exactly what is stated on the bill since the tip has already been added. If you don't look for this you will end up double tipping.

Also, if I have had mediocre or poor service, I will dispute the addition of the service charge. Restaurants will remove it if requested because
a) the charge is not legally enforible in UK
b) restaurants do not like people complaining in loud voices.