Comment by mytailorisrich

> Then got myself the local residence card one morning

Well, exactly. Some countries require/required registration and residence card. That did not exist in the UK when it was in the EU, you just showed your passport/ID card when you needed to prove your right to be there (basically once in a blue Moon). Even now EU residents don't have any physical documents.

The National Insurance number @pdpi mentioned is unrelated as everyone has one once they work and an appointment is not always required to get one, and you can actually start working before you get one.

If you work as an employee there is also usually nothing to do regarding tax.