PH wrote:....
The requirement is for you to mark the paper, there is no restriction on how you do that, the polling station should have been closed down, firstly for getting that so wrong, secondly for even looking at what you were doing in the booth, thirdly for giving you an unrecorded slip - it takes a court order to link them but there's a requirement that it's possible.
https://fullfact.org/europe/it-safe-vote-pencil/
The only reason the use of pencils is encouraged is the thick easy to read mark they leave and that they don't smudge.
We may be talking cross purposes (sorry) as you were discussing pencil vs pen but I did find something interesting trying to find my own local council candidates and strayed into EU Referendum results for my constituency
The interesting bit is the rejeced paper because the individual made a make that identifies him/her (presumably signed or something).
Ian