I get so angry when I read things like this (about you being in excrutiating pain due to huge endometriomas and just being shoved on a ridiculously long NHS waiting list!) I have been in this same situation twice!
I'm so sorry you are suffering like this. I really had to fight to get my last surgery within a reasonable time. So I will share what I did... It might help?
First I rang the surgeons secretary and asked to be put on the cancellation list (which they automatically did apparently with everyone!) then I rang a few times to see how the list was progressing (I did end up annoying the secretary a bit, but felt I had a right to know)
Once I found out I would not be having surgery within 18 weeks from my GP referal (which is the current longest wait the NHS is supposed to make you wait for treatment....... Please google the the 'NHS patients right to treatment') I emailed and then rang and spoke to PALS..... Which is the Patient liason Service at the hospital you are attending for treatment. I said how my surgery was going to be a longer wait than this and said I would be looking at being reffered to another hospital with shorter waiting times (you can ring round to find out the waiting times of hospitals nearer or further away)
I also stressed my pain levels, and said I was close to being unable to work, and had already given up all social life as I simply could not function.... And that I would loose my job if I was off sick for months. Also fearing for my fertility due to my ovary being swamped with a rapidly growing endometrioma.
This prompted them to get back to me saying that I would not breach the 18weeks, they promised I would have surgery before this!
I did have to ring up again when the surgeons secretary said they were wrong to promise this (rolling my eyes!) and literally that somehow made my surgery happen just 2 weeks later.... 17 weeks from my initial referal.
So Defo read up about the NHS waiting times and get in contact with your PALS. I know women who, after this, have got their MP involved successfully as well.
If you are worried about the cyst growing, you should be able to get your GP to arrange a scan. If it has grown bigger, you might be able to get your GP to write to the hospital saying it's more urgent you get surgery sooner? My GP was going to help me by doing this.
Message me if you want any other info...... I just hope you can get surgery much sooner than you expect. I have had good relief from the removal of endometriomas! (Largest was 10-12cm, and they managed it via keyhole)
Good luck and sorry it's been such a long post! (I hope you are still awake reading it)