Is there a way to contact the Heathrow emergency services on your mobile? If there was a true medical emergency and you couldn't find an internal phone (or never knew about them) would calling 999 get the message passed onto the internal emergency services?
Dialling 999 or 911 and requesting "Ambulance" would get you through. As soon as they established your location, the Airport Paramedics (who are employed by London Ambulance Service) would be deployed straight away and back up requested.
As far as technology is concerned - your alert system may happen one day. t the moment, most places - airports, shopping malls etc - will provide the bare minimum required to be legally compliant. In fact, Heathrow and Gatwick spent extra in having a "kick strip" built-in. Most disabled toilets on the outside only have a pull cord alarm.