Yes, a lot of movies actually use slangs and slurs that are present in everyday life to make them sound more authentic. It is going to sound less clear in terms of pronunciation.
What sort of movies are you into? I think I've not watched movies in a while, but the last few X-Men movies don't have a very thick modified English accent, if I remember correctly.
How about watching talk shows, though? I watch shows like QI and 8 Out of 10 Cats. Jimmy Carr enunciates his words very well, even though the rest of the people in his panel speak in accents usually (with Sean Locke being the clearer one to understand).