I've never understood the value of having the air ambulance for places like Brixton which are relatively close to hospitals - with all the landing and taking off and then landing again surely it would be quicker for an ambulance to drive the 1-2 miles to King's or do the air ambulances have specialist equipment?
HEMS (Helicopter Emergency Medical Service - London's Air Ambulance) carries a trauma Doctor and a specialist paramedic, they can get anywhere within the M25 in 12 minutes. Once on scene they have two options - package up the patient and fly to the heliport on the roof of the Royal London Hospital where a full trauma team await them, or ground assist by road to the nearest hospital/trauma centre (Kings is one) which means they go in the ambulance with the crew and get a lift back to the chopper later. Air ambulances from other services (Kent/Essex etc) can convey patients to King's by landing in Ruskin Park and then loading the patient into a London Ambulance to transfer round to the A&E.
HEMS carries advanced medical equipment for things like amputation, intubation, immobilisation etc and the staff to use them. The Doctor can also give more drugs than paramedics.
The HEMS paramedic in the control room scans incoming calls and will further interrogate callers to see if a call needs HEMS, it will be dispatched if required, or an ambulance crew will arrive at a call and then request them.
At night and on days when the chopper is offline (maintenance, bad light etc) then two cars are used with drivers. On calls the pilot always stays with the aircraft.
ETA: One of the main procedures HEMS Dr's perform is
Rapid Sequence Induction for badly injured patients which involves administering a general anaesthetic.
As well as HEMS there are also BASICS Doctors around London (it's a nationwide scheme), these are Doctors with advanced life support/trauma training (some are Casualty Consultants, some are GP's) who are willing to come to the scene of an accident to administer drugs or perform specialist procedures (mostly it's nasty trauma where more pain relief is needed) from their work or home address. If HEMS is busy or a BASICS Dr is closer then if a crew requests a Dr the nearest one will be rung to see if they are able to attend.
HTH.