It wasn't all that long ago that Scotland was largely in favour of the Tory Party. Maybe an eternity in political terms. But then basically Thatcher happened, and it's pretty clear that, for at least some Scots, memories or stories of her premiership will take ages to get over.
Shortly before the Scotland referendum, someone had drawn a chalk mural of Thatcher, just a little up the hill from the Meadows towards town, with the delightful caption "feel free to spit on her face". I didn't take the opportunity myself, but it's a pretty low blow to write or do something like that.
But anyway. SNP candidates standing in English constituencies would be interesting. I suspect it would be a long game, though, as eg the first time round it's hard to see their campaign getting too many results from a start of exactly no support at all. And then by the second time, we'd be out of the EU anyway, and a pro-EU party would, I'd have thought, struggle in England under those circumstances.