I think its a question of different protocols rather than greater formality. In the UK, government offices are positions rather than titles and are never attached to the name. The holder is addressed by name or position, but not both. So it is correct to address the Prime Minister (who is, after all, supposed to be first among equals) as "Mr Cameron" or "Prime Minister", but not as "Prime Minister Cameron" or "Mr Prime Minister".

US usage is apparently different, and perhaps this rubs off when they address lesser mortals such as "Chef" Ramsay.

American children do seem to brought up to show greater respect to their elders, however. If TV programmes are to believed, it appears common for children to address their father as "sir", at least when they're in trouble, something no British child would dream of doing.