Tbh, I don't think he is trying to undermine anything, he's making a cheap joke on the supposed unintelligibility of Scottish accents, which is about as ha-ha as it sounds and deservingly twattish. We could make similar jokes about people called Quentin, few of whom I'd hazard are born on council estates. Leastways, not a lot of Quentins in my school, though it would have been a early death sentence so possibly they didn't survive the first day. There's plenty of parliamentary sketch material featuring May's robotic mannerisms. Admittedly it's a bit funnier. No one would make a robot that useless, of course. Some of them can negotiate stairs these days (though that's a lot of steps below negotiating Brexit).
I've no idea about Nationwide adverts and death threats, but if there's one thing social media has taught us, it's that there certainly are a lot of fuckwits out there (and I'm getting less convinced that there is really any upside to social media). I suspect they aren't actually planning to kill someone over a twee song (leastways, not unless it's the Birdy Dance) but lack any filter. Of course, as a cyclist, I'm familiar with people threatening to kill me for any mild to negligible inconvenience I might have caused them. Give these shitbags any kind of anonymity and power and they'll abuse it.