If your relocation to London has been organized as part of a relocation jobs package then your employer will apply for all of the relevant visas and government requirements. If you are moving to London on your own, or have an accompanying partner who wishes to work, you will have to apply for a National Insurance Number quite soon after you arrive.
If you are going to be living in London for any amount of time, then you’ll have to apply for a National Insurance number as soon as you arrive. It is as important as having a Social Security number and will have a huge impact on your life in London and the facilities and services that you are allowed to use. An NI number is a sort of identification number that the government will use to identify you and your contributions to tax, and other services while you are living in London. It is important in a number of ways including signing a lease on a London apartment, opening a bank account and using the health care services and hospitals.