I encourage anyone who feels this way to spoil your ballot paper. This sends a different signal than not voting: if there were lots of spoilt ballot papers this would be received differently to low turnout.
There is no "message" - the message is to vote for the least worst, or start your own party.
Do they even count spoiled ballots? They certainly don't report them.
I guess in theory, someone who spoils their ballot has "turned out" to vote, so should be counted in the turnout. So it should be possible to work out the number of spoiled ballots from the difference between the turnout, and the total of votes cast. It would be interesting to see if the news media report it so the difference is zero.
See "Rejected ballot papers" https://www.electoralcommission.org.uk/who-we-are-and-what-w...