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...