You will pay in working tax credits then. Or just the traditional way, a gob in your food or the cabbie "accidentally" taking you round the back streets.
I always tip 10% unless the service was noticeably lousy. Many restaurants assume that their staff will be tipped a certain amount, and pay them lower wages in consequence. It should not be that way, and we need strong labour protection laws to ensure that workers are fairly compensated. But until that is the case across the board, I am happy to help where I can, especially as some working in these jobs struggle to make ends meet, or are doing it on top of another job that doesn't pay enough or doesn't guarantee enough hours. Working in the catering industry is extremely gruelling labour. I know, because I've done it.
For those people in the thread who worked in fast food joints or other places where tips aren't common... well that sucks, but how does taking away others' good fortune make your bad fortune any better? It doesn't. That's just mean spirited.
Is tipping really a thing in UK?
I'm not giving small loans to anyone.