Attached Link: http://www.bbc.co.uk

Don't worry about the accent as much as the choice of words and how they're put together - Mum vs. Mom, Lift vs. Elevator, etc. If you want your characters to sound British, especially "average, middle class British," I'd suggest going to the BBC website and reading around there, especially the travelogues. (The travelogues tend to be written in a conversational manner so you'll pick up a lot of current phrases and phrasing.) If you want to hear different British accents, they do have video/audio clips all over the place as well. Cheers!