This was my thinking. The fine would suggest there is sufficient irregularities to void the election result.
Although there is a police investigation going on in parallel (which the Electoral Commission is feeding into), and perhaps the outcome of that will force a by-election - morally at least, not sure what the legal protocol is.
Farage has said that if they re-run Thanet he’d stand again and, in the circumstance, he stands a pretty good chance of winning…