Hey, I've been reading through this topic and to OP, as another autistic person - yep, English Lit can be a right pain in the arse. As someone who sat their GCSEs last year (and nearly missed an exam because I'm an utter genius), here's my adivce:
If you've already mentioned it to school that you may not show up, then they may remember that when you don't show up - it really depends on how much the student managed you mentioned has to do with the exams in school. I suppose you could call school on the day of the exam to say that you can't do it, but it really depends on who answers the phone - do you know if it'll be someone who knows and understands your circumstances? If so, then they might not reschedule it - I'm not sure if they're legally obligated to, but they might. If it won't be or you're not sure, then I wouldn't risk it.
Regardless of what happens with your English Lit - if you're possibly stressing over all the possibilities, keep in mind it's just one day, once you've missed it, no one will ask you about it again - I wish you luck for your other exams.