If people knew the election date in advance and then bet on the date, that is clear cut "insider" gambling. You know the result and you bet on it before it's common knowledge.
I am not sure about betting on event where you don't know the outcome. To what degree can he influence the outcome?
And he bet on himself to lose?
I suppose you could run a poor campaign. But he has more to gain by actually winning.
I am sure the Tory media will lap it up and see it as an equivalence.