I'm not an employment lawyer, but to the best of my knowledge this is incorrect - mass redundancies of 20+ people need to be notified to the government in advance, and failure to do so carries criminal penalties.
P&O seems to have escaped this requirement (at least for now) owing to maritime employment being a special case and jurisdictional issues
https://www.rsmuk.com/ideas-and-insights/legal/mass-redundan...
For >=100 redundancies, notice to the Secretary of State is required and not giving notice is a criminal offence, by both the company, and also any officer/director/similar who is involved in not giving notice: https://www.legislation.gov.uk/ukpga/1992/52/section/194
A level five fine on the standard scale is (since 2015) technically unlimited, so it's not a toothless law. Not sure if anyone has even been prosecuted though.