I'm sorry, and I am sure that the situation you are in is making it hard for you to think straight. I've seen people in that situation. But nothing you are describing is unlawful. They don't have to give you jobs that match your skills. They have to give you jobs that you can do. And you are not suggesting that you can't do them, in fact to the contrary it seems you are saying that you are getting the work because others are not competant to do the work, which suggests you are! You aren't entitled to a trial period unless you are being made redundant, and there is no suggestion that you are being made redundant. And please, please be cautious about decsribing work as "menial" - that sounds like arrogance, even if that isn't what you mean.
Let's try getting at this another way. You have a long notice period - does that mean you are quite senior? The two would ususally go hand in hand. Can you not go off sick if it is making you ill and then retire? Can you ask to retire early?
If that isn't possible then you need to try to explain what they are doing that isn't lawful. I'm really sorry but everything you are describing is not showing anything unlawful. Your managers have left, and you cannot be left without a manager, so you are being moved to other work. That may not be the work you want to do, but if it is deemed to be at the same grade and on the same terms, and your role is not at risk, then they aren't obviously changing anything fundamental. They perhaps ought to be discussing this with you, but I'm finding it hard to pin down exactly what they are doing wrong from what you are saying. By wrong I mean something that the law says they can't do.
If you can't do that then all you have for a grievance is that you don't like your job / managers / HR - and in all honesty they are going to throw that back at you and say it's tough luck.
This is a bit of a reach, because nothing you've said suggests this is the case. But would this be a public sector TUPE? With older workers the public sector is often very reluctant to let people go even when they don't really have a role for them any more because of the adverse impact on pension etc. It can actually cost more for the employer to let them go, but at the same time reduce the pension the individual gets.
I wish I could come up with something better here, but I am not seeing anything that is going to substantiate a grievance other than the fact that you aren't happy. And that won't do. I won't give up trying if you don't - but you need to give us something that is unlawful or meaningful that can frame a strategy.