I think the best place to observe seriousity in action is in queues/lines for security at airports. If a security guard plucks an American out of the line at JFK for a thorough check with the wand, everyone’s demeanours will be solemn – the checker, the checkee and all members of the checkee’s travelling party. In a similar situation with Brits at Heathrow, it’s more likely there will be smiles exchanged, a few chuckles and perhaps even a bit of banter about bombs in bags between the checkee and their fellow travellers. There’s almost a social requirement to make light of the situation.
Of course both Americans and Brits love to laugh and joke, but humour is more likely to extend into serious contexts for Brits. Somehow they seem to provide irresistible opportunities for irony, which of course makes it tricky for Americans (and many other foreigners). How can they know whether we mean what we’re saying or whether we’re just joking around? Heck, in British conversations, I’m not sure whether I mean what I’m saying or just joking around a lot of the time.
So anyway, I can’t see seriousity making it in the UK. But I mention it because it’s something to bear in mind if you’re a Brit trying to speak ‘merican.