Thank you for asking. It's been a stressful few weeks so I totally forgot to update.
I have appointed both friend and dd guardians in my will.
The solicitor said that this will happen automatically if I die, so nothing needs to be done at the moment bar this and they will simply take over here and step right into my roll. They can apply for PR after I die as well and a court would definitely grant it for dd, and almost definitely for friend.
The will and guardianship will also mean that benefits can be transferred pretty much automatically, as this is the proof they will need.
Ex won't be informed in any official way at all, there's no legal obligation for anyone to do this, and he doesn't need to give permission for me to appoint guardians in my will. SS also wouldn't automatically be involved, only if someone raises a concern, as they would with anyone else (I contacted them about all this but they weren't especially bothered or interested) however the school are well aware of the situation and know friend and dd well so they wouldn't have any concerns as such.
If ex should find out and kick up a fuss the kids will remain in their home, being looked after by their guardians and ex will have to go to court and essentially prove that he will be the better option for the dc.
I have so much evidence that he isn't, his house isn't big enough, the kids are settled here, they would be asked for their opinions and wouldn't want to move to be with him either. I doubt he would want to spend the money on court anyway, I really think he just wanted to hurt me and make me stress.
His PR won't be removed, it would take a hell of a lot for that to happen, and the stuff he has done would never meet the threshold for that, but it's essentially meaningless in the situation I'm in as dd and friend will have primary care so he would have to take situations to court if he didn't agree with something, and they won't entertain him objecting to anything just to make life difficult.
I've secured everything as much as I can for the kids, and I know ex can't screw things up without a fight, so I can concentrate on myself and my health for the moment, and hopefully none of this will ever be needed.
If I should get a terminal diagnosis then friend and I won't have to be married for any of this to happen, but he will be moving in here as it will be financially feasible for that to happen, and then he will just remain living here, which Ive given him the right to do until the youngest is 21, he has planned what to do with his home so he isnt losing out financially. We have looked at how much benefits will pay out, and how to maximise that with careful placement of my insurance money (nothing illegal and all solicitor approved), I know that probably sounds scroungy but it's really not fair for friend to be taking a massive financial hit.
So basically I'm as organised as I can possibly be, and I feel a lot more relaxed and confident about it all after speaking to my solicitor and getting my will all watertight.
My next round of treatments starts in a little over a week, and I'll be having an operation fairly soon too, hopefully, so fingers crossed it all goes well and I've just stressed and created a massive dramatic scene over nothing because I'll be fighting fit by Xmas.
Thank you all again for taking the time to help me. It really was a relief to discuss it all, and helped me clear my thoughts and get my head sorted out.