Speaking as another Brit I really don't get it. Leaving aside differences in pronunciation, the only thing that separates the two dialects is a few minor spelling/vocabulary differences. I don't remember ever bothering to change from US to UK English on sites that support both. The one exception might be if dates are involved, in which case it is confusing to have the US order.

Go to the US, and ask for a 'glass of water' anywhere. Honestly, hardly anyone will get it first time - and usually you have to break down and ask for a 'wadder'.

Same with Butter.

There are some major differences that aren't obvious - us brits tend to be more flexible with mangling of the pronunciation (because everyone here does), but in the US they like it their way or the highway, so to speak.

I live in the US and that's not really my experience, but I was thinking more of written communication, where the differences are very small. (I did say "leaving aside differences in pronunciation.")