Yep, in the UK it's almost always a 12.5% service charge. Almost always with terrible service of course.
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.
it's mostly around being interrupted mid sentence, in my experience (and it's certainly the thing that annoys me the most)