current rate about 15% as the vicar says (I can remember when it was 12% and occasionally you're informed that the going rate is 17%, so it's not static). I only eat in steakhouses and suchlike where I've never seen a headwaiter; they just give you a credit card bill with a space marked 'tip' and another marked 'total' at the bottom. Round the figure up to something convenient and fill it in under total; they can work out the exact tip themselves. Feel free to make the tip a lot lower if the service wasn't good enough - it's how they learn.
Sometimes waiters grumble that they don't actually see any of this tip, it just goes to the house, so you might prefer to leave cash on the table; but that's really a problem for the waiters to sort out with their employers, not something customers ought to solve for them