It has always annoyed me that many bills in restaurants and cafes here in London "prescribe" a tip of 10%, sometimes even 12,5% under the item "Service". Being that one often pays by credit card this additional amount often slips through under the radar when one pays the bill. It is my understanding, however, that waiters and waitresses do receive at least the minimum wage here in the UK and other than e.g. in the US tips are not supposed to be their only source of income.
I don't like this method and have taken the whole amount off the final bill when the food or the service have been sub-optimal. This is how I learned that in some places the "tip" goes to the owner anyway and that only a fraction may reach the person who served you.
This has lead The Independent to start an intiative called "fair tips, fair pay". The newspaper seems to have been successful, as it reports that the government now backs the initiative. Regretfully, however , there will be no now law in this respect, but an approach is favoured by which customers are informed about the existing practise e. g. by way of a remark on the menu.
For my part, I have decided to only give cash as a tip and to give it directly to the person who served me.