Taken from the good old book:
Category B
Flights operating for search and rescue or other humanitarian
reasons. Post accident flight checks. Other flights, including Open
Skies Flights, authorised by the CAA.
Air Ambulance would be Category A:
Aircraft in emergency (e.g. engine fault, fuel shortage, seriously ill
passenger). Aircraft which have declared a 'Police Emergency'.
Ambulance/Medical aircraft when the safety of life is involved.
Full list of categories here:
http://www.caa.co.uk/docs/33/CAP493Part1.pdf
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DF