In some countries, it will create problems in getting access to public services if people are not registered. Under EU law, they could be there for three months without registration. Some countries say that after those three months, you have to register. If you have not done that, and you want access to welfare benefits, they would say, “Well, you haven’t registered.”
It has always been the case that countries could do that. The UK could have done that, but has not done that. Having people who are not registered now creates a difficult problem for the future, because non-registration will have the immediate effect that you become illegal. Even in the EU, it is not, “If you haven’t got registration, you aren’t legal.” You might not have access to certain services, but you are not illegal and you are not deported on that ground.