As far as I'm aware, you can only be served alcohol at 18 irrespective of whether or not you are also being served food. Otherwise, pubs and clubs would be falling over themselves making sandwiches to get 15-16 yr-olds in.
However, I would hazard a guess that if you are 16-17 and you are going out to a restaurant for a meal, they will probably serve you alcohol if you are nice to the waiter because chances are you are not going to get blind drunk and puke on the bouncer or anything.
Found this information as well:
Below is a summary of the main age restrictions relating to drinking in licensed places:
Under the age of 14 children are not permitted into the bar area of a pub unless the pub has a children's certificate. In this case they can enter if they are accompanied by an adult.
At 14 you can enter a bar or pub but only if the landlord agrees and if you drink soft drinks (this can include low-alcohol beer).
At 16 you can buy beer, cider or perry (made from pears) in a restaurant or eating area of a pub where there is no bar, if you are ordering a main meal.
At 18 you can legally buy drinks in a pub, bar or off licence. If you are having problems getting served in pubs and you are over 18 you could apply for a proof of age card from the Portman Group, an organisation set up by the drinks industry.
By the way,whatever someones age is,to drink on Licensed premises they must have the consent of the Licensee.The Licensee has the absolute right to refuse to serve anyone for no reason whatsoever.