This is another article written from Hillingdon Hospital (next to Heathrow airport) in the Journal of the Royal Society of Medicine:
Indemnity is the security against damage, loss, or injury or a legal exemption from liability for damages. Under current British law, UK registered airlines operating out of the UK have no legal obligation to offer indemnity to Good Samaritans. It is at the discretion of the airline whether they offer this legal protection out of goodwill. British Airways, Virgin and other major carriers will indemnify a medical professional against legal liability, unless grossly negligent, that might arise from their assistance with an onboard medical emergency. In contrast, the US adopted legislation in 1998 in the form of the Aviation Medical Assistance Act, which includes a Good Samaritan provision.10 This protects passengers who step forward to offer medical assistance from liability unless they are guilty of gross negligence or wilful misconduct. This act dictates what happens on US registered aircraft only and the terms are irrespective of current airspace. Indemnity from the airline is only applicable if the airline requests your assistance, usually verbally via the tannoy. It does not apply if you spontaneously offer assistance, and you should therefore wait for a request to be issued. If assistance is requested directly by the passenger, as could occasionally happen, it is pertinent to inform airline staff of the situation and clarify that indemnity will be provided.
If an airline does not offer indemnity to the acting doctor, medical defence organizations will provide insurance in the event of any legal proceedings, unless the potential claimant is bringing the action in a court under American or Canadian jurisdiction. In reality, it is extremely rare for a passenger to take action against a Good Samaritan.