The article says that in one case they can evidence it, but an individual MP isn't a police force. At any rate, the reasons behind the design of elections is to pre-emptively block fraud because proving it retroactively is hard. Hence the private ballot box, polling stations, the reluctance to use computerised/internet based voting and so on. People have to believe the system is inherently robust. Why, because what sort of hard evidence could be generated for those sorts of problems, and at what scale? People registering with their friends/family addresses so they can tip votes outside where they live is inherently hard to obtain large-scale evidence of.