Ruddy, I've honestly never heard of that one in 15 years teaching! we do occasionally disaggregate an INSET day, but definitely not for our convenience.
The last one I can recall was because we'd all otherwise have had to come in on the first Monday of the hols. Many of us would have preferred this, but it wasn't allowed - can't remember why - so we did 5 separate twilight sessions instead. It was ghastly.
Our HT is a stickler for us not being seen to gallivant - he insisted that non-striking colleagues didn't go for a pub lunch last strike day, & having agreed to my department being off-timetable for a day for essential controlled assessment moderation, he made us book a conference room at an out of town hotel rather than just all meeting at one of our houses.
But I've absolutely never come across INSET being used for shopping, even if the time was made up in twilight. Tbh I'd be mightily pissed off if my own dc's school did it!