Because the culture is to make up the wages by tipping, even in the uk that is true. That keeps a lot of power in the consumers hand.
As the tipping rate in the uk continues to decrees, we are seeing more and more companies building in wages into the price of the menu. Which means we have no control over it and receive worse service.
what kind of improvement are you supposed to see exactly?
as your tipping after the meal unless you're a regular the tip will not affect your service at all.