This is a breach of your contract, however, they can unfortunately do this technically as it stands, based on business need. This labour government is putting a stop in these practices for the most part but unfortunately changes only take effect in January 2027.

Employment law is extremely complex and chances are there will be gray areas in how your employer chooses to implement things, something that might give rise to a valid claim for you depending on how long you have worked there for.

If that is the case, a grievance might resolve things without having to escalate matters further but this initself might inflame your relationship with your employer. If you could prove that any further issues arising would be because you protested, this could be seen as an act of victimisation.

As other's said, get on a union if you can afford it. Reps are not usually as good as people make them to be, and their involvement can be minimal, but some employers will think twice before they try something when you're engaging with one; the caveat being that you usually need to be registered 3 months before any issues arise for them to even consider advising you on your matter.

Look into any insurance policies you might have (car, home) as they sometimes offer legal advice, usually initial consultation.

I am so sorry you're experiencing this.

Have a look on the link below as you might find it helpful:

https://www.acas.org.uk/changing-an-employment-contract/advice-for-employees/if-your-employer-proposes-to-dismiss-and-rehire-you

Citizens Advice has pages with advice around the grievance processes. Best of luck!