Former school withholding UCAS reference, now panicking.

Who is this person's line manager?
Ring and ask to arrange an appointment in person with the guidance teacher and if that isn't possible then ask to arrange an appointment with their line manager as you want to raise a formal complaint. Writing a letter isn't going to get you anywhere especially with the Christmas holiday coming up.
Alternatively are you able to get in touch and on good terms with any individual subject teachers in the school? Asking if they'll provide a UCAS reference directly as an independent referee might be a way to bypass the person who is ghosting you.

Worse case scenario I’d submit the applications pre-deadline without a reference. It’s more important to be an on time app and your choices can ask for it later. Relying on a bit of goodwill from them but better than resigning yourself to clearing.

Thanks for your reply, in a worse case scenario (which I won't think about until mid January), should I email the unis back and ask if I can submit my application and get a reference at a later date or shall I just click the box which says no reference needed and hope for the best? What would be the cut-off for getting a reference in a conditional offer if they were okay with that as I know i'd be able to get one from the open university when my module is complete but by the time the EMA is marked, that might not be until the end of July/start of August depending on when the results are released, of course I hope I can get the school to add mine to UCAS before the cut off date but that's looking unlikely to be honest.

I contacted my former guidance teacher to request a UCAS reference the week the schools in my area went back after the October holidays (contact was made on the 29th October). She agreed to get one of my former teachers to provide the reference and said it would be ready in about 2 weeks so I emailed back after 3 weeks and got no response (She is usually on the ball with emails so surprised me), tried emailing again the following week but no reply and again no reply the week after that. To add to my troubles, despite contacting them a good 2 and a half weeks there deadline to see if my reference was complete, it is now past the internal deadline for my former school so i'm worried even if they have done the reference, they won't be able to input it so I can submit my application.
She also asked me to link my application to the school using the buzzword (both in the normal way and the applying independently but requesting a reference way) but I have tried that multiple times and it has been rejected every time.
I have since tried to make contact with her twice by phoning/emailing the school office but I all I get is the admin support assistant saying she has forwarded my enquiry and no further reply. I'm down to my last line of defence which is to put a letter in the post, I'll do that today and send it first class so it's hopefully there tomorrow but I can't see that working as the office staff probably just open all the mail.
I have contacted my preferred universities this week and are awaiting their replies but I doubt they will be able to help.
I left school to do an online open university module for a year so I would get the open university to provide me a reference however they can't provide me with one until I complete the module and I want to avoid taking a gap year at all costs.
All I want is advice to help me further as I don't want to not be able to go to university over my former school never giving me the reference they agreed to provide, I know i've got until 29th January to get my application in before the equal consideration deadline but I would rather sort it sooner rather than later so i'm not panicking because the deadline is approaching.