If their objective is to damage the UK they can do that without breaking it up. For example, for example by fostering violent Scottish Nationalism, or even just sowing hatred between the Scots and the English, or between Scots who want to remain the UK and those who do not..

Its quite unusual for separatist movements to both remain peaceful themselves, and not to be violent suppressed. Even other western democracies have not managed, nor did one other part of the UK for a long time.

My other nationality is Sri Lankan so I have seen how bad it can get. I do not think that will ever happen in the UK, but even a very small number of sufficiently nasty people can do a great deal of damage.