I think the special case of the totally unnecessary by-election is important here. I would not expect the other parties not to contest the general election. I don't like the "X is an attempt to distract from Y" framing, but this really is a distraction from the "sudden revelation" that Farage has really dodgy financing.

Also, Farage is the ultimate "establishment" candidate. He would be the same level of obscurity as Tommy Robinson or Restore if he didn't have a regular seat on BBC Question Time. All of his money comes from a small number of rich donors. He's appealing to anti-immigrant sentiment in a constituency that has hardly any immigrants.