Two contrasting stories..... and why tips are a minefield.
My son worked in a local modern food restaurant mainly used by upper working class or lower middle class diners. He could double his wages from cash tips.... especially those from large groups on a night out. The owner did not believe in service charges as she thought that decent waiters would bring in the punters.
My daughter worked in a restaurant in one of Leeds' chic areas where many young 'professionals' dined. They tended to view the mandatory service charge as the tip and rarely offered any extra. Given the cost of the meals and the charge at 12.5% the staff were not convinced that it was passed on in full, or anywhere near.
Both of them were on nat min wage as their base rate.