You can just not vote. It has the same impact on the results of the election.
Here's a case from last week of a spoiled ballot giving someone a majority of one: https://www.theguardian.com/politics/2019/may/03/ballot-pape...
Was that the result the voter intended? Probably not, but who knows? :)
This was a constituency where the previous incumbent had had some problems with his younger GF
Not bothering to show up at all doesn't give any clear message.