When you go to eat out you are not really paying for food, you are paying for someone to make your food, the general ambiance and for someone to wait on you. It is your choice to use this luxury and that is why it is customary to tip.
I agree with makingeverydaycount that the above statement is bollocks and also that the ambience, luxury and service is built into the price.
For example, DP and I went to Pizzaexpress for a quick lunch a couple of weeks ago. We had Tesco vouchers that covered a lot of the cost, otherwise, it was extortionate. Over Thirty Five Pounds for two pizzas, nuts and olives, and a single large bottle of beer to share. For middle of the road chain restaurant food!
We could buy nearly a week’s food shopping for that! The same amount of food in the supermarket would cost between £5 and £10 for equal amounts and quality.
Those of you who think tipping is necessary would have probably rounded up to £40, so about £4 or so in a tip? We were in and out a little over an hour and in that time, our waiter will have spent no more than 5 minutes serving us, and waited on other tables at the same time.
Assuming that the other tables were the same, and they probably weren’t as some would have had more people, more food or more drinks, but even if a waiter served 4 tables per hour (and I would expect that it would be more than that) and everyone tipped 10-15%, that would add up to at least £10-20 per hour on top of wages. Surely waiting staff don’t earn that much?
In the end we paid the portion of the bill that wasn’t covered by the vouchers by card and left a £2 coin as a tip. I think that is more than generous for a few minutes interaction over an hour, for someone who would have also earned at least NMW plus other tips. But a lot of people on here would say that that is mean and I shouldn’t eat out if I don’t want to spend a week’s food shopping money on a single lunch out.
Upthread, someone said that they waitressed many years ago (over 30?) and never came away with less than £25 for a 5 hour shift, suggesting that she sometimes received more than that.
As a comparison, I remember that just 20 years ago, in my first job as a lab technician in the NHS, carrying out tests that determined what type of medical treatment people received, so at least as responsible and important as carrying plates of food 
, working full time 7.5 hours per day my full time annual salary was £6600.
So the maximum I earned for a single shift that was 1.5 times as long as the waitress’s shift was, wait for it £25.38.
So in the UK, I feel no guilt about not tipping 10-15% for mediocre/average service in restaurants, and I stand by my opinion that waiting tables is a similar level of job to retail and cleaning, and no where near as important as care work, so in no way deserves tips to enhance normal pay.