Work in retail, do I have to do more overtime over the x-mas period or will refusing put my job at risk?

Hello everyone, I work in retail and do 16 hours a week which is what I was hired to do. The main reason I dont do anymore hours than that is because I have autism and working more for me leads to burnouts, meltdown and all round deterioting of my mental health. Now that its coming up to the christmas period some of my colleagues have started to get their christmas hours through and even the part timers are getting like 30 hours which is something that just wont work for me. Since autism is a disability I think that should give me some protection but I am worried if I end up saying I can only stick to my contracted hours they will get rid of me My contract does say the following: >All employees are required to be flexible in their working arrangements. Your hours of work, shift/ working pattern or rest breaks may therefore be varied on a temporary or permanent basis >In this respect, the Company also reserves the right at its absolute and sole discretion to increase or decrease your minimum agreed committed hours of work and/or change your working pattern (including days, hours, weeks, meal and refreshment breaks), on a temporary or permanent basis, as necessary to meet the operational requirements and/or trading and shift patterns of the store. I have also only been in the job a few months so I dont get the protections I would past two years. What would be my best course of action if my christmas hours are to high when they come through?