1 week/9 weeks notice???

I have a contract with the previous owner of a dental practice where I have been working for the last 3years.
The current owner changed my job title without a notice, demoted me and the whole practice changed (my team got fired, new people hired). This all happened when I was in hospital. Now I am back and know that they want me gone (well, I presume).

In my original contract it says I have to give 9weeks notice prior to me leaving (due to the employment period of 3years). Now - the new owners don't seem to have a copy. They are pushing me to bring the contract to prove there is one.
I am not keen on staying for 9weeks. The place is going downhill.
Shall I show them the contract and make clear I want to resign in hopes they will let me go immediately and pay me off?? Or shall I pretend I can not find the contract, give a written notice with 1week and offer them negotiation if they need me to stay longer?(just to be covered in case they have the copy of the contract)

I live in the UK

OK. I bit my tongue, wrote a professional and polite letter in which I stated the 9weeks period. I also stated that I wish to terminate the employment immediately if no obligations from their side. Wish me luck! 9weeks would kill me under that management.

Produce the contract. That way, you can use it if a legal claim arises, and they may let you out of it anyway.

I know that here in the US the contract would still be legally binding, and it would be enforceable (in most states).

Produce the contract and at the same time say that you wish to leave on xxx date.
If they argue say that the contract although binding in law works both ways and that if they refuse to let you go then you will sue them for breach of contract and unfair dismissal as your rights are carried over from one employor to another.
Say that you will also contact the other employees and remind them that they can also sue if thay don't let you go.
Good luck.