Complex sentence: London's Air Ambulance, is an air ambulance, also known as a Helicopter Emergency Medical Service (HEMS), which responds to seriously ill or injured casualties in and around London, England.
Split into simpler sentences: London's Air Ambulance, is also known as London HEMS (Helicopter Emergency Medical Service (HEMS)). It is an air ambulance service which responds to seriously ill or injured casualties in and around London, England.