The UK doesn't cease to exist though, it just shrinks. Plus the USSR fragmenting and Russia (as the main constituent part, the nuclear power and the country the independent republics were happy to acknowledge as the continuation of the USSR) becoming the successor state is pretty well-established precedent for what happens when states fragment, whose legitimacy Russia probably doesn't want to contest too strongly...

General disruption in the UK would help the Iranian government a little, but I managed to click on one of the accounts before it was suspended, and its most popular tweets received very interaction (and were pretty banal statements of independence support indistinguishable from stuff thousands of completely normal Scottish people posted) I assume their attempts to seed wilder rumours were low effort and had very little success.