There is no law that you need to leave a tip and YANBU if you don't leave a tip. Carry on eating out and just don't leave a tip. There is no obligation to tip. There is no way you should stop eating out, the price of food is high enough already and priced to cover their overheads including wages.
I am sure that they would rather have your £14 than not have it as they will soon go out of business if everyone stops eating there.
You will get comments on here telling you that you should leave a tip, you can't afford to eat out if you can't afford a tip blah blah blah. Just ignore them. We don't all live in the same world with the same income.
I always select no as I won't be pressured into leaving a tip and don't always leave one, it depends on how many in the party and what the total bill is etc and most importantly what the service was like. If I do leave one, it will be left on the table or given to the staff depending on the venue.
I don't buy into the "it's a low paid job" thing because it is NMW the same as most shop staff round here and I don't tip the girl in the local bakery or butchers or whatever.
and yes, I have been waitress and bar staff in the past and tips are great, but they should not be expected, they are just a nice bonus of the job