Just to add a statistic I read in the various Property related sites and resources I read.
http://www.aplaceinthesun.com/10% of the UK population live abroad.
This is a mixture of economic migrants, retirees for the main part.
As mentioned, I think statistically, most UK or British Indigenous (sp?) would probably want to stay here even if such a 'open US/UK agreement' ever happened, those who are perhaps from immigrant families may want to look into this, I'd be off like a shot if it was to come about! however as I know it won't, I currently favour Canada over the US, but only just!
Everyone is different though, so there'll be allsorts of answers from the general public if you could ask more people here rather than just UKY members.
Cheers, DtM! West London & Slough UK!