Taxi to the gate, call for a doctor, ask CC to assist if doc onboard or AED onboard, that is the quickest option. If an FO can't do that they should not be in the seat.
Wot, at Heathrow? Sloppy Joe, I would hate to be you or your conscience if your Capt pegged it under the circumstances suggested and you did this. On the other hand I would LOVE to be your prosecuting lawyer. I would crucify you for that. If I had sold you life insurance I'd withdraw that instantly, for fear of what you'd do when you finally woke up to reality...
Jeez! Talk about the milk of human kindness - or not.