An employer can pay an employee who does not have a NI number. They are placed on an emergency tax code until the NI number comes through. As long as you have a legal right to work in the UK (which EU citizens have for years to come), then it's not an issue. I think this is purely an administrative exercise rather than an exam. Once you have a NI number, you are able to claim benefits so I think the job centres just need to make sure you're there to work rather than just claim benefits.