Interesting how it’s changed over the years. I was a deputy returning officer a few elections ago and the official line then was that pencil was the accepted implement because it was less likely to smudge and spoil a ballot.

Of course, you’d have to make sure the pens were all the same colour, otherwise it could be used to mark ballots (I. E. I use my green pen to prove I voted for so and so). That could circumvent the secret ballot.

makes sense, i was a little worried when i saw a pencil in the booth but this makes me feel better.

The UK election commission recommends pencils as there was a worry that ink could become invisible in the ballot box if there was an organised effort to replace the pens. But that’s the provided writing implement. I don’t think there is any rule for bringing your own. And I don’t think the counters have time to invalidate a vote because of marking type.