You shouldn't expect much from a British burger, except perhaps food poisoning. Burgers in the UK can mean anything from just a meat pattie all the way up to a top-of-the-range gourmet burger with attitude. Except more down the plain meat pattie end of the range from greasy spoon cafes and take-aways. Most often this kind of burger is a meat pattie served inside a bun. No salad, no extra ingredients, no sauce.
On the higher end of the scale, check out the Gourmet Burger Kitchen, a more recent innovation importing Antipodean burger values. In these places you get quality meat, buns with a little bit of give and actual other ingredients! Amazing!
A little add-on. "Amazing" isn't the right word, the burgers at GBK kick ass! TimeOut magazine had a burger joint review in one of their issues in 2004. Their final result was "GBK - one burger to rule them all!".