Hi Tinstaafl,
Category A
Aircraft in emergency (e.g., engine fault, fuel shortage, seriously ill passenger). Aircraft which has declared a "Police Emergency". Ambulance/Medical aircraft when the safety of life is involved.
Category B
Flights operating for SAR or other humanitarian reasons, post accident flight checks. Other flights including Open Skies Flights authorised by the CAA.
Ref: Manual of Air Traffic Services Part 1, p 1-32.