I am a strong believer that tips are to reward those who have gone over and above what is expected as a minimum, and entirely at the customer's discretion. As many others have said, I always leave a few pound coins rather than adding onto a card payment.
My brother will always leave a tip of 10% of the bill in a restaurant regardless of the quality of service, and never tip anyone else. I take a rather different view- presumably the staff are being paid to do their job, and if I choose to, I can tip any amount I feel appropriate- and I extend this to other situations as well, usually leaving a tip for hotel cleaners in the room before leaving, and a Christmas tip for your binmen can prove a good investment if you later have a bit of extra stuff to get rid of that won't fit in the bin.
