The rules governing valid and spoiled ballots for the UK can be obtained from the electoral commission at https://www.electoralcommission.org.uk/sites/default/files/pdf_file/UKPE-doubtfuls-booklet.pdf
2.2 The (A)RO may only reject a ballot paper on the following grounds:
• it does not contain an official mark
• a vote for more than one candidate has been given
• it contains a mark or writing by which the voter can be identified
2.3 A ballot paper must not be rejected because the vote is:
• not marked in the proper place
• marked other than by a cross
• marked by more than one mark
So as previous precedent has established, drawing a penis in a box counts as a valid vote if the drawing is entirely contained within the box.
So as long as you clearly vote for only one candidate and don't add anything which might identify you, the vote should be valid.
Any contentious votes are put aside for a decision by the Returning Officer (I believe it is done with observation by the candidates in case they want to argue any individual decision).
I don't know whether a ballot which clearly votes for one candidate but has other text written elsewhere would qualify as contentious, and can't tell you whether it would be seen by the Returning Officer (and candidates) or not.