The first time I applied for one I was denied because allegedly I didn't show any proof of employment. Of course I couldn't because I didn't have a job yet. And I found it really ridiculous that they require me to give them proof or payslips when I hadn't started working yet. And it seemed to me that before they employ you you must first have an NI number. I was like sandwiched between two mountains. Luckily, my first agency accepted me without it and they just asked me to apply for one later on.
Now once you start working in the UK you pay for national insurance on top of the taxes. But I noticed that I only am getting deductions for National insurance and not for taxes. Maybe because I haven't really worked for one whole year but I'm afraid that when they start deducting taxes from my pay I would be getting only half of what I am netting now.