We were talking international not internalID check from the airline is for the airline’s benefit rather than anything official - revenue protection only, to prevent ticket resale. Flying internal with a decent airline you’ll be asked for neither. International flights will have passport check by airline too, but again that’s to prevent them getting fined by the receiving country and has no emigration status at all - HMG doesn’t care who leaves the country.
Mate flew from England to Scotland at the weekend and didn’t show any ID, as I’ve done many times in the past.
Even ferries and tunnel require API for those leaving the UK now