I don't understand the motivation to spoil your vote. First past the post is the shittiest voting system but the rational response is to vote tactically instead, perhaps reduce the majority of your disliked incumbent. Even if you can't overturn a majority, MPs on smaller majorities may be less arrogant, and less likely to vote for unpopular policies. But sometimes you do overturn a majority. It will happen lots in this/next year's election.

I don't think any politician gives a shit about the numbers of spoiled ballots, they literally don't look even once at those numbers.