Would this not have been a immigration issue (the locked room thing, not the original event) ?

Presumably SOP is to detain all inbound pax until passports etc. can be checked, no matter what previous events have meant them being there !

I suspect that a large part of the frustration and upset could have been prevented had a BA person rocked up and said "Sorry, ladies and gentlement, we have to ask you to remain here until Border Force check you into the UK. As soon as we know anything, you will know it too."

I also suspect that no-one had the gumption to do so...