Russia was allowed to inherit the USSR seat on 3 conditions:

- It took on all the sovereign debt from the newly independent nations.

- It relinquished nukes that were left behind in Ukraine.

- The United Nations collectively agreed to it.

I don't think any of those things would happen in the UK's case. But of course it doesn't matter what you or I think. It only matters what _Iran_ thinks will happen if Scotland gains independence.