Re: validity of the Brexit vote
Are referenda covered by the same laws as elections?
As far as election fraud - yes. AFAIK so far campaign fraud has been used to invalidate elections up to mayor and councillor level. There was a London borough which had its mayor removed a couple of years back for that (forgot which one).
I am not aware of a single instance of parliamentary election (as the closest equivalent to a referendum) to be invalidated as a result of campaign fraud.