NI means National Insurance number. Those who are in student UK visa can able to work twenty hours per week with minimum wage of £6 per hour. But to work we need to get number called National Insurance number. First thing we need to do is call one land line number and attend interview with them. Just will be asking simple questions like the purpose of your visit your university and local address where you residing in London and your passport details. Once you successfully finished your call they will register you for NI number and they will reference number and land line number which can be used for further clarifications. After seven days you will get one form, please fill the form with correct details and send to them. This service is free of cost. With fifteen days you will get NI number then you can start working. One important thing is don't disclose this number to any one. Because it may cause problem when you apply for Visa extension.