I have two different experiences.
1 - I have worked myself in a restaurant's kitchen when I was a student. Our manageress (she was beautiful) used to count the tip for the evening and line it on the table in equal proportions for all the staff, waiters, waitresses and the kitchen staff. And I know what those 5,6 pounds mean for a student who are not living with family and is miles from home or anyone who has people to take care of.
2 - Second experience when I moved to the other side of the table. I am a kind of person who would never leave place without leave few quids. But yes always cash even if I pay bill using debit or credit card.