My attitude to tipping somewhat changed last year when my teenage daughter took up a part time waitressing job at a pub food chain whilst studying for her A levels. Ever wonder why a lot of waiting staff are teenage boys and girls? Because minimum wage for under 18s is currently £3.89 an hour (was £3.69 when she was working). A six hour shift on a Saturday would result in bringing home barely 20 quid, and that is 6 hours of constantly on the go and on your feet. Any tips received during a shift, (shared out by a formula, but the largest shares going to "table hosts", as well as bar runners, food runners, and kitchen staff) made a significant contribution to her pay.
It also appears that older employees on higher minimum wage rates and also on essentially zero hours contracts would be the first to be stood down as the restaurant quietened down, leaving just the cheapest and youngest staff left.
I suppose there are two responses to this. Either avoid using them completely if you feel that they are cynically exploiting their staff; or if you do eat there, ask yourself how they can manage to offer 2 mains for a tenner, or kids eat free deals, and leave a little tip for the serving staff - unless the service and experience was truly poor, then hold it back as is your right.