extra a level year

i am currently in year 13 and i have chronic fatigue syndrome (ME) and it's getting to the point i cant manage to go into school - i am constantly in pain and fall asleep in most lessons, and sleep most of the time when i get home and don't get any study done. i cannot function like i used to be able to and know i won't get the grades i need to go to my unis because of this, i don't even think i'm going to be able to go into school anymore at this rate. i was wondering if my sixth form would be likely to let me take another year for my a levels - if i had some time to rest i would be able to recover and study more, however my health is so bad at the moment i'm struggling just to attend and stay awake, and i have memory loss of what we learn in lessons. is there anything that i could use to help my argument, as i really want to study law at a russell group uni and know i have the potential-i achieved all 8/9 at GCSE and have high predicted grades, but without an additional year there is no way i will be able to do this. i also missed lots of year twelve and a few weeks of year 13 due to various illnesses and infections and haven't been able to catch up on this and it's adding up. does anyone have any experience of this and know what i can do? thank you