As a trade union rep the answer is ...........
Your employer can change your conditions of service (contract) providing they give statutory notice of their intention to do so in writing.
There is no requirement to consult with you or a trade union, but most do.
The notice period is one week for every complete year of employment up to a maximum of 13 weeks.
This is most often used when a company changes ownership and is covered at some depth under the TUPE (Transfer undertaking & protection of employees) regulations.
Most companies do the 13 weeks as easier to change everyone's contract at the same time rather than having some on the old and some on the new contract
Sorry, but your stuffed, you have the legal right to refuse the new contract, but we call that resigning.
An intelligent, concise response that directly answers the question posed. Shame you don't post more frequently.