This is because nobody carries any cash around anymore. And it's not socially acceptable for waiters to ask customers how much they want to tip. You just tap your card on the portable terminal.
As for the service being terrible, it can be. But British people generally don't like to be interrupted during their meal. So the waitstaff have a tougher job to be present but not harassing.
As for the service being terrible, it can be. But British people generally don't like to be interrupted during their meal. So the waitstaff have a tougher job to be present but not harassing.