I am a little bemused that you find his accent hard. I talk this way ( come from the same part of London, met him in a pub once but thats another story!) and I have never had a problem being understood when I've been in the States.
I have noticed though that you seem to have more problem with the men and kids, the kids had to repeat themselves alot. Most of the time I ended up speaking for them which they loved as you can imagine.
We speak faster and I have to admit to becoming quite frustrated speaking to Americans, it takes for bloody ever to finish a sentence sometimes.
The colloqualisms are becoming more transatlantic I think. We use alot of yours thats for sure.
I am desperately in love with Tim Roth so I'm probably biased but he just wouldn't be the same with an American accent.
I hate Hugh Laurie's American accent, it grates on me. Maybe because in real life he's quite posh, I don't know. He's just better with his original accent and very charming, bless him.
Well thats my two pennies' worth.
Laters S