a) They need to have registered EU citizens already in UK before they can enforce new regime
b) they don't have a new system up and running.
So here we have the Whitehall workaround...
They can still enter the UK in the same way.
They can still get jobs, access services or housing in the same way.
But after 3 months they have to ask the Home Office for permission to stay for another 36 months.
In practice it means very little though.
That means it's just not enforceable.
But there will be a whole load of people who arrived after March who either won't be registered (for good and bad reasons) or who fail to meet new criteria?
So what will Home Office do? En mass deportations of EU citizens?