what separates the U.S. from Britain linguistically?

[All Along The Watchtower]

False.  Mass media is not accelerating linguistic convergence.  Regional dialects may be getting somewhat less apparent in the UK, but that's largely a result of increased geographic mobility.  Meanwhile, dialects are becoming increasingly more distinct in the US, by and large, with the continued decrease in speakers of some East Coast dialects (e.g., Charleston) being more than balanced by large changes in the Rust Belt and California.

I was under the impression that TV (and to a lesser extent, films) desire to have most "normal" characters speak with a flat, midwestern accent was having a rather large impact on our "lesser" accents.  Hence why people in California and the Rust Belt now sound like they just got off the bus from Des Moines.

Accents in the western part of the country are just less distinct because those areas are more recently settled and have fewer natives.

+1