Yep, in the UK it's almost always a 12.5% service charge. Almost always with terrible service of course.

That's at worst a couple euros per person, very far from a 10%-20% upcharge.

It depends on the country. It is not the case in France for instance. I don't know exactly what the law say, but you shouldn't have to pay for anything that you didn't order, and you shouldn't pay more than what's written on the menu.

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.