Does anybody really think that things they write on a spoiled ballot paper (none of you deserve my vote, you're all liars, I don't trust any of you, etc.) will be taken seriously or acted upon by the candidates or their parties? How naive.
If you have a specific complaint about the incumbent, or you don't like the particular policies of any party, why not tell them before the election?
I know somebody who is standing today, although I don't share his political views. He says that when people say to him, on the doorstep, that politicians are "all a bunch of crooks" or similar, he asks them to explain why they think that he in particular is a crook, with specific reference to the crimes he has apparently committed.
He also says that people who tell him that they plan to spoil their vote are often the type who fixate on a single issue specific to them, which they naively think they can campaign for by whining on a spoiled ballot paper and which are more about selfishness than the overall manifesto pledges of any of he parties. Examples include:
"My granny has to wait three months for a hip replacement because the NHS doesn't have enough money, and by the way I'm paying too much tax." (yes, and you're too stupid to see the connection)
"My child benefit has been cut, and we're spending too much money on benefits." (see the correlation, do you? No, of course not)
"There aren't enough healthcare assistants or carers in my area, and also I think there are too many immigrants" (yes, the same people who come to Britain keen to work in exactly those jobs).
"I want free childcare and I think that schools should provide free wrap around care from 7 am to 7 pm. I pay far too much tax." (you really believe in that bottomless pit of cash, don't you? If you want Scandinavian levels of state provision, get ready for Scandinavian tax rates)
Don't spoil your ballot paper. It's the equivalent of taking your ball home when you're a kid. As others have noted, choose the least worst option if you don't support any of the parties.