It seems like the word "biometric" was missed in a LOT of information, although I could swear I have been fingerprinted about five times for all of my EU permit stuff (the 6 months entry and then the paper residence card), so not sure why I need to get printed yet again but hey ho.
Those who want UK "visas" have to do this all the time, to prove who they are. I think it was physicskate who explained this well when she said to think of it like when you want to use your phone.
Entering the border at LHR yesterday was the same as always other than the agent asking me "and you actually live here". Why yes, yes I do. Do you want my tax statement showing my 40% tax rate too?
The border officers ask questions as they record infomation presented. They also have the power to refuse entry, cancel visas.
You working wouldn't prove you are lawfully in the UK because you are not on a UK work "visa" but are an "EU Extended Family Member". EFMs have no rights under EU law and the EU merely asks the EEA countries if they will "facilitate" Extended Family Members; which some govenrments will and some won't and they can change their minds on this if they want to, as the UK has recently done for some who had hoped to be EFMs in the UK. This is why Extended Family Members must hold a valid Residence Card from the UK to be lawfully in the UK.
e.g. if an EEA citizen is working in another EEA country and decides to end the relationship with their unmarried non-EEA citizen partner, that person immediately loses their right to live and work in that EEA country as they are only an Extended Family Member and their RC is now invalid. If a non-EEA citizen remains in the UK on an invalid RC, then they are an illegal overstayer and if they work they have committed a criminal offence. Whereas if they were married to their EEA citizen they would be a "Direct Family Member" and these
might retain rights in that EEA country under EU law, subject to certain stipulations laid out by the EU.