When you work here, you need a national insurance number. As long as you have none, different organisations will create different national insurance reference numbers for you. It might be that your employer organises your meeting for the National Insurance Number which will take place in a Job Center (which is what the AMS (Arbeitsmarktservice) is in Austria), but you might also (have to) do it yourself. You can only apply for a national insurance number once you’re in the UK.

More information about the National Insurance Number can be found here: https://www.gov.uk/browse/tax/national-insurance

Here you can find out how to apply for a national insurance number and what documents you might need: https://www.gov.uk/apply-national-insurance-number