Earlier quoted context omitted.
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.
Citation very much needed. I have never seen anything in UK employment rights law that says "unless the employer is in trouble". 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.
[1] http://www.legislation.gov.uk/ukpga/1996/18/section/98 [2] https://www.bto.co.uk/blog/refusing-to-accept-a-wage-cut-%E2...