I hope this is in the correct board

I am in the process of applying for the unmarried visa with my British partner. There is a question asking for my NI number, but since it has been over 6 years since I worked there and I didn’t receive a card at the time, I do not have access to my number. The time it will take to have it sent is to long unfortunately and it won’t let me add “not available” in the field. Obviously if I say I don’t have one then that is untruthful.
Any advice on how I should possibly proceed would be greatly appreciated.
ADDED: I also noted an old thread about the same issue was posted but the resolution/best way to proceed, was never posted and it is now locked unfortunately.