[Rainbow Six]

My thoughts...

Died - disease. One would assume that she would have the best medical care possible, so whilst possible but imo unlikely

Died - nuclear strike. More likely than died from disease, particularly if she stayed in London. imo whether she would have stayed in London or not is an interesting question. It's possible that she would have chosen to do so particularly for morale reasons, in much the same way that her father and mother remained in London throughout the Blitz. Personally I could see a case for staying and not staying (or departing before the strike, possibly to a relatively remote location such as Sandringham or Balmoral), so I'm 50 / 50 on this one, although I do think the fact that she had abdicated so was no longer Sovereign might strengthen the case for staying, effecively acting as the nation's figurehead whilst the King quietly slipped out of the Capital as a mater of practicality.

It's also possible she may have escaped the initial attacks and died in the strike on the Government's wartime command post that is referenced in the SGUK.

Alive. To a large extent depends on whether one thinks she survived the nuclear strikes or not. If she does then I think it's likely she will still be alive and in a secure location that is well guarded.

Held by someone. Not impossible, but I think unlikely, I think we can safely assume there would be sufficient security around surviving members of the Royal Family - particularly one as senior as the former Monarch - to ensure their security. iirc several Foot Guard Battalions are unaccounted for in GDW's order of battle.

(Incidentally, the thought has just occurred to me that the Royal Duties Force might make an interesting article in its own right).

Unknown. One could argue that no matter how well guarded some catastrophe could have befallen her and she is missing...if that were the case we could assume that enormous efforts would be being made to try and locate her.