The GDP PPP per capita in that report is 77 for Portugal and 80 for Greece. Scotland has 98. This is a scary difference.
Spain has a large economy with unequal economic performance between regions. The four region in Scotland have 98, 89, 159, and 80. The 7 regions in Spain have 89, 121, 126, 83, 101, 74. Mismanagement could mean Scotland ends up in the 74-89 range.
My point is big parts of the UK are already in pretty low range so if this is your big scarymonsters thing it's a big scarymonsters thing in the UK already. The Northeast is already Portugal. Wales looks even worse. And yet the assumption remains that the UK is just better at things. I'm not seeing that.