There has been new allegations of "breaching of rules" by some pro-Brexit groups in the Brexit referendum. This new allegations join previous ones that electoral laws have been broken (also here). Some have called the result "illegitimate"; others "invalid".
Now, legally, in the UK, when can a referendum result be officially pronounced as invalid, in the sense that it result is not binding, and must be rerun?
If this is never the case, then there is all the incentive for parties to cheat. In my view, the legitimacy of democracy itself is in question if cheating in a democratic process cannot invalidate it, and its only consequence is financial penalties to the cheater. Thus, I imagine there must be a rule which puts a limit to this incentive.
In the UK, there exists the Electoral Commission, which rules regarding referendums include "certifying and announcing the result." Surely certification must be based on some definition of legality of activities of parties involved?
PS: there is this related thread, which is about what happens if rules are broken. My question is a bit broader, asking if there is any way in which an official organisation like the Electoral Commission has the authority to qualify a referendum result as "invalid".