This is pretty common knowledge. Though not on the side Cameron would have wanted.
https://en.wikipedia.org/wiki/Russian_interference_in_British_politics
Russia funds separatist groups across Europe. Just search and you find ones like these:
https://www.todoalicante.es/english/council-europe-claims-russian-20250408100414-nt.html
Why would you agree they interfered with Brexit but not the Scottish Independence?
Divided we fall, so of course our enemies fund every level of separation they can. And to be fair, I bet we do the same to them.