Excelcius, you make some very good points. Although I would agree that teachers don't get paid anywhere near what they are worth, neither do waiters. I'm not familiar with the pay scale in the UK, but in the US, wait staff, except in the fanciest of restaurants, often get paid minimum wage, sometimes less, out of which they must pay taxes not only on the wage itself but also the tips - plus they have to divide their tips with the buspeople! In some cases that leads to paychecks with zero or negative balances! So even if you don't like the system, it's hardly fair to make your "statement" about it by not tipping someone, therefore leaving them with virtually nothing for the service they gave you.
Having been on that side of the fence, I'm reluctant to "stiff" (not tip) them, unless service was truly abysmal - even then, I'll just leave a smaller tip than one would normally expect.