If you are an expat and intend to work and subsequently pay taxes in the UK, you must apply for a National Insurance Number (NIN). Each NIN is unique, it is made of letters and numbers and look like this: QN 18 23 67 A. Once you obtain this number, it will be your permanent personal number for as long as you remain a resident of the UK. To understand what exactly is a NIN and why you need one visit: https://www.gov.uk/national-insurance/your-national-insurance-number
While you do not need a National Insurance Number to rent a flat, you do need one even if you are seeking a job or have yet to find employment. This number is also necessary if you intend to use the National Health Service, so it’s essential to apply for a NIN promptly upon arrival in the UK. It is important to note that even if you are self-employed you need a NIN to begin paying taxes.
In order to obtain a National Insurance Number you will be required to attend a pre-scheduled interview. You must call Jobcentre Plus at 0845 600 0643 to schedule an appointment. After verifying that you do not currently possess a NIN number, Jobcentre Plus will confirm the date, time and location of the interview and will tell you what information/documentation to bring with you. The interview is short and the documentation you will usually need is your passport and proof of entitlement to work in the UK. Again, Jobcentre Plus will confirm the exact documentation. After you have satisfied the interviewer’s requirements, your number and card will be issued within a few weeks.
The following website is a useful resource for expatriates new to the UK and needing to obtain a National Insurance Number: https://www.gov.uk/apply-national-insurance-number.