If there is a medical problem we contact Medlink in Phoenix by HF via ops, Sat phone or acars.
We give all the symptoms and we are passed to a specialist in that field and how it relates to flying.
We are then advised by Medlink on the action to be taken.
By following their advice we are indemnified against any legal action from the passenger.
I'm not sure about LGW but at LHR to get paramedics to meet the aircraft you are obliged to declare an emergency.This has to be a PAN as the words "Medical Emergency" alone are no longer recognised, or so I'm told.
BA's FCO's dictate that "when medical assistance is required on arrival, a medical emergency (PAN) should be declared.