How can I change my visa to a Covid extension without the school's work permit cancellation letter?

Hi, I was wondering if anyone could give me some advice please. I have recently had to leave my job at a school in Chiang Mai and have to return back to the U.K. as soon as possible. I am on a work permit with a 1 year visa extension, the school informed me on the day that I handed in my notice that they would cancel my work permit with immediate effect. I understand that I now need to go to immigration to change my visa to a Covid visa extension to give me the time to sort things out for flying home from Thailand, however when I asked the school for a copy of the work permit cancellation letter I am getting no reply. Also they have the work permit in school, as it was still in process so I only have a digital copy of the temporary permit. Is there anyway that I can change my visa without this documentation from the school? (Sorry if this has already been covered and I’ve missed it) Thank you