As I recall, McSween isn't too bad. I agree, pork 'haggis' isn't haggis. M&S should do one of their sultry-voiced ads - "This isn't just haggis.... In fact it just isn't haggis."
As you can imagine, here in the Highlands you're spoilt for choice. Mine was my preferred one, from one of our local butchers, the wonderfully named Hastie and Dyce. I'm not sure why I like that name so much but I just do. If a detective duo drama was ever set in Inverness....
Yes, you can't beat the local butchers really. My father was from Dunscore, near Dumfries, so I knew the haggis of the local butchers, Jim MacFadzean in Dunscore, now no longer with us, or John Houston in Dumfries, John is gone too (was a friend of my father) but his son still has the business, https://www.jbhouston.co.uk/
It's a shame your M&S ad wouldn't work to a wider audience, it's genius!