That is fine, everyone gets confused with these rules, you are not the first.
If you are exempt from worker authorisation and want to be employed here, you will not need to obtain a work authorisation document. As someone who is exempt, you will be able to take employment without restriction. You can obtain confirmation of this rightly applying for a blue registration certificate (but this is not compulsory).
Further information about exemptions can be found on the UK Border Agency website at www.ukba.homeoffice.gov.uk/sitecontent/documents/policyandlaw/ecis/chapter7.
You will be exempt from the requirement to obtain authorisation to work in the UK if:
you have been working with permission, and without interruption, in the United Kingdom for a period of 12 months ending on or after 31st December 2006. This may be the case if, for example, you are already present in the United Kingdom as a work permit holder or in some other category that confers permission to take employment (for example, if you are here as a student and have been in part-time employment continuously for 12 months);
You are able to undertake employment before the arrival of your blue card as you have already been in the UK as a student and undertaking work. You therefore do not need to wait for your blue card to arrive.
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