If it was found that Russia interfered in some capacity, would/could anything be done?
One of their most high profile cases: https://en.wikipedia.org/wiki/Erlam_%26_Ors_v_Rahman_%26_Ano...
I don't think either of these apply for nebulous "external interference", but certain actions such as running unlawful electoral advertising should count.
Vote leave have been fined and referred to the police: https://www.electoralcommission.org.uk/i-am-a/journalist/ele...
BUT: these apply to elected positions. Referendums don't really have a strong constitutional place in the UK, and the Brexit referendum was "non-binding": the result did not legally oblige anything specific to happen.
Brexit is a choice of the government. No-Deal Brexit is also a choice of the government, made two years ago when sending the A50 notification without a viable plan.
I'm not convinced that the idea that Russia interfered is worth taking that seriously though. You don't need to go very far to find real people who are very pro-brexit. IMO it's the newspapers (rupert murdoch, etc) that had more influence, but I'm not sure what you can do about that.
For the Brexit referendum, there is no need to invalidate it, as it was an advisory opinion poll rather than a binding public vote for the government. The UK government could ignore the results or call for a new referendum if they wished to.