If Farage wins, things continue exactly as they were: Farage the MP of Clacton, being investigated for undeclared financial gifts. With no serious counter-candidates, even getting 99% of the votes would be completely meaningless.
If Farage loses, well, he'll have lost to a bin, meaning his political career would be finished.
And anything besides a massive victory would only hurt Farage. Prior to this he was seen as a likely future Prime Minister. If he gets a 55/45 victory against a bin in a constituency which heavily favors his party, who would ever take him serious?
If the other parties contested the election, they would've legitimized his stupid "I'm getting a mandate from the people" move, automatically making it way more of a mess if his undeclared financial gifts turned into being expelled from the House.
The only winning move is not to play. The other parties know this, and by treating the Count as a completely legitimate candidate the media are guaranteeing this.