The UK is starting to think about its online adult laws, at present you simply should avoid breaking any existing uk laws.
Mainly the following;
Extreme (anything that could hurt or threatern the life of the models)
Depictions of Rape, forced *** and similar
Urinating scenes
Underage scenes, or scenes that make it look underage (such as teacher - schoolchildren)
Enjoy

But again consult a lawyer don't use my advice as the be all and end all.