"Meaningful employment" the key to access the welfare system is the possession of a National Insurance number.The UK did. People from the EU could not access the welfare system unless they are working, or treated as working (e.g. they were working, got pregnant, and will return to work). The "right to reside" for benefit entitlement was clearly defined.
The rules referred to meaningful employment, though. So someone in full employment but without a permanent home wouldn't necessarily be in breach of the rules, as the courts found. And it's perfectly possible that a minimum wage job in London would be insufficient to rent somewhere on a long-term basis.
I think I saw a TV documentary, the newly arrived person spent a few hours collecting scrap metal, weighed-in the scrap at a scrapyard for money leaving with cash and with a receipt, with the receipt as evidence the person registered as a scrap-metal collector and lo and behold issued with the coveted National Insurance number, I think we can guess the rest!