BOAC I suggest you modify your knowledge of non ETOPs twin engine flying.
If you are non ETOPs you must be within 400 nms of an airfieild. The met conditions at that airfield do not matter. Therefore you may be considerably furthur away from a field you can land at than if you are on a proper ETOPs route with flyable weather. Of course airmanship would stop you doing that but what about comercial pressures and "pressonitis".
eg, you may find yourself between Iceland and Greenland on one engine. Sonderstrom and Kef are both out so you flip a coin to decide if you head for Ireland or Newfoundland. Insanely horrible but legal.
I have avoided technical terms so any one can under stand my point. Even a four engine pilot!
As Uncle Roger said years ago ETOPS:
Engines Turning Or Passengers Swimming