I asked my dad about spoilt ballots...

[Llamallamadingdong]

He is a (now retired) returning officer and he told me this:

1. All spoilt papers are separated and declared “doubtful ballots”

2. Prior to the final count result being announced, ALL candidates must review ALL doubtful ballots. The reasons are two-fold ensuring every paper is accounted for. It also provides candidates the chance to look at the nature of the spoilt papers and the messages written on them as they desperately try to claw back some votes.

It causes a LOT of issues marginal seats where recounts may be demanded. Those candidates want EVERY ballot paper scrutinised closely again.

So although one individual spoilt ballot is not reported on, if large numbers of the electorate spoil their papers, it will be national news (as was the case in France a few years ago).

Well , in practice noone is going to be able to identify a person by their handwriting if they don't even know which constituency it is!

Each constituency counts its own ballots.

Here’s the wording from the Electoral Commission - ballot is spoilt if

“The voter can be indirectly identified by any writing or mark on the ballot paper
2.15 The (A)RO is not required to investigate the matter or require evidence to be produced to identify the writing or mark, but the (A)RO should consider any evidence that is given to them at the time.
2.16 Where there is doubt about the identity of the person who marked the ballot paper, (A)ROs should allow rather than reject the ballot paper.”

So maybe it only counts if you actually sign your name.

Don’t the polling papers actually have an identifiable number on them so you could be identified by officials in any case?

Yes but you would need a court order to match up the numbers with the voters.

(In theory)