It's hell, isn't it, when they get to this stage?
As long as she's happy, give her extra food, perhaps see the vet and find out whether it is thyroid deficiency or something else.
In my experience, cats know when their time is up, and make it quite clear to us who love them when they have had enough. Like you, I hope she slips away peacefully when the day comes.
To think that London, or anywhere else for that matter, does not belong to any one demographic