[quote RaptorInaPorkPieHat]@JamieLeeCurtains I have to admit I've written (political not rude) messages on the ballots before.... but only when I'm spoiling them because I don't agree with elected Police Commissioners 
I've not stooped to drawing willies yet...[/quote]
Trust me, they do get read!
Such ballot papers have quite an interesting journey. They're opened out flat by the vote counter. If there's a scrutineer watching, and they're a quick upside-down reader, they see it too.
Procedure is that such ballots are handed to the counting table supervisor and looked at, and passed to the Returning Officer. The question has to be answered satisfactorily, 'is this or is it not a clear intention to vote?'
The Returning Officer periodically bundles up all the dodgy ballot papers with penises, messages, emojis and whatever, and makes a preliminary judgement. He or she then calls up the relevant candidates and their agents for a huddle in the corner of the counting hall.
A consensus is then reached as to which ballots are to be considered officially 'spoiled', and which are 'clear intentions to vote for a candidate'.
It annoys me when posters on MN say that writing on your ballot paper is a waste of time and just goes in the bin, because actually quite a few people do see it, including potentially very senior councillors and a council's CEO.