Some of us operate by the principles of universalism. I.e., if everyone tried this scam and got away with it, Priority Pass would be forced to discontinue the benefit.
It's pretty obvious the intent is not for you to sit there and keep swiping your card, or leave and come back 12 times in an hour. One credit per visit. Each time you go to the bathroom doesn't make it a new visit.

And the restaurant probably isn't getting 15 GBP per swipe. Priority Pass pays them a fraction, as the idea is that you will end up spending more than 15 GBP. And you probably should be tipping in cash if there's decent service.