In the beginning I used to reply with the truth: ‘I’m sorry I don’t know her’. But I soon realised this could be perceived as quite rude.
I therefore have lately changed tack and have begun trying to draw out a longer answer, ‘ummm yeah I’m not really that into the royal family I’m afraid but ummm yeah she seems fine and everyone in England gets a bank holiday so that’s cool’.
At this point most excited-American-strangers look a bit disappointed. I think they were expecting a bit more from my English accent, something along the lines of me jumping up and down and demanding champagne to toast the happy couple there and then.
Most excited-American-strangers then try and prompt me to respond more adequately by reiterating how excited I should be: ‘but the big days getting close isn’t it, surely you’re excited?’/ ‘do we know who is designing her dress yet, surely you’re excited?!’
I regretfully shrug: ‘I’m sorry I really don’t care’
Nine times out of ten the excited-American-stranger is seriously confused by now, ‘but Kate is the biggest thing since Diana, surely you care?’
At this point I tend to start shuffling slowly away sometimes calling apologetically behind me ‘she does have really nice hair though!’