HEMS has 3 types of dispatch.
1. Immediate. A certain limited list which if the paramedic in ambulance control sees, will task the aircraft immediately.
2. Interrogation. The paramedic has the ability to listen-in to any 999 call as it happens from one of the call-takers. He/she can then ask extra questions about the casualty's condition. Based on those questions then task the a/c.
3 Crew request. Any member of the ambulance service (in London or the surrounding counties) can request the helicopter and it will be tasked as if an immediate dispatch. Counties such as Hertfordshire Kent & Surrey regularly request the aircraft for accidents involving Neuro injuries. They are aware that a neuro patient will end up in London anyway as there are no neuro-surgical facilities in these counties. These are the cases where time really is important.
With respect to the hospitals used, it depends on the patients injuries and the type of treatment required. The Doctors brief is that a patient should go to the nearest, most appropriate, hospital for their injuries. Generally if a suitable hospital is within 20 minutes ambulance drive (blue lights) they should go by road. (Bear in mind that the average speed in London is only about 12mph even on blue lights!). Otherwise go by air - usually to the Royal London. The decision is entirely up to the Doctor on the helicopter based on their assessment of the patients injuries.
There are only two hospitals in London with multi-trauma facilities - The Royal London (where the aircraft is based) and Kings College Hospital. However these hospitals do not have specialist burns units so a burns patient could end up anywhere a specialist burns bed is available.
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