There's no requirement for the provider to give you a cheaper package. You are most likely going to get a cheaper package by moving provider. The OFCOM requirements are outlined below. It uses the words 'best deals'.
https://www.ofcom.org.uk/about-ofcom/latest/media/media-releases/2020/companies-must-tell-customers-...
Further explained in the FAQs at the bottom of this article
https://www.cable.co.uk/providers/guides/end-contract-notifications/
The initiative kicked off around Feb 2020 which is why you are receiving the notifications from that point forward advising you that you are 'out of contract'. You would have been 'out of contract' before this but there was previously no obligation to tell you. I received such a notification in Jan 2021.
It mentioned that my contract had ended and what I was paying monthly. It advised I might get a better offer with another provider. It listed two comparable current packages at comparable prices. I was offered a cheaper price on a current package by a slight downgrade in services. It also advised what a new customer would pay for the same downgraded package.
I think for your argument to go any further you would have to find which packages VM currently have on offer, with comparable features to your existing package, which you have not been offered in the annual notification.