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)
12.5% service charge at the bottom.
Tips are appreciated but not expected. I's say that if you are used to the American way, tip what you would normally tip above your baseline. For example if you normally tip 15%, and 20% for good service, then in France, don't tip for average service, and tip 5% for good service.