I don't really have problem with UK progs. Certainly as far as accents go. Mumbling can be more of a problem for eg the Jamaica Inn series last year. Accents were fine but mumbling spoiled it. I also have probs with some US films. Again mostly because of mumbling and once you get atuned to the movie it isn't so bad to make out accents - but there are some actors I really struggle with. There was one film with Jesse Eisenberg that I stopped watching simply because I couldn't understand him. Not just mumbling but mumbling at speed.