Earlier quoted context omitted.
Yes, if you decide not to sign the new contract, and they fire you because of it, you can take them to tribunal for it. It's illegal.
IANAL but it's not always illegal. If a company has a choice between going bust and imposing pay cuts on everyone then that is perfectly legal.
If the business is in danger of going bust because it can't afford to keep people on then it may need to make some of them redundant (and redundancy rights will then apply), but that's totally different to unilaterally changing the contractual terms of employment for the worse.