Where's this accent from?

There isn't a standard British pronunciation. It is just like in America, there are different accents for different places. There is however received pronunciation which a lot of actors learn. Mackenzie Crook has a Kent accent. In P.O.T.C. he speaks with a slight cockney accent (this isn't actually too far removed from his own accent, but it is just a lot lighter than his own voice.) Mundungus Fletcher also speaks with a cockney accent, however his is more of an East-End London one. Hope this helps =)