They may need to fly critical care patients to other areas, if the ICU beds in London are overwhelmed. The issue with COVID-19 is that the critical care patients can need 2-3 weeks on a ventilator-which means once you are hooked up, then it can be unavailable for that time.
I saw the French shipping ICU patients to Germany yesterday. The Italians were medivacing patients by helicopter to other parts of the country too.