Hello there Sime. Good day and thank you for reaching out. I hope that our fellow DJI Co pilots located in UK can give out the best recommendation with regards to this matter. Thank you and have a nice day.
It's your Operator ID alone, NOT the other one. see https://register-drones.caa.co.uk/labelling-drones
You could always hand write one, but use capitals, on a sticky label and then add sellotape for protection.