It's kind of a wired way of betting to me. I wonder what else they place bet on that sounds very wired. Well, in common sense if the body was so close to the prime Minister, he probably must have gotten a tip from someone on the inside about the election date. That would be the only way he had gotten it and made a bet which is cheating obviously. But it might be a mistake though as no one can actually tell if that's what happened. He might just be blamed falsely just because he works for the prime Minister. He might just be like every other gambling who would have won the bet on similar occasions but the condition of his work just gave him another issue to fight over.
But couldn't he have bet using another identity? I mean, he should ask his wife or someone close to him that he trusts to put the bet in his place so that his name is not involved in the matter. This possibility confirms the stupidity of his doing so, no matter how tempting the amount he might have won.
At the same time, this makes me wonder whether the bodyguard is authorized to see any sensitive state information. Is there a law that prevents people who hold duties in the British administration from placing bets in general? If the accusation is proven, the bodyguard will be accused of using sensitive data obtained as part of his assigned work, and placing a bet is evidence of that and not an accusation in itself.