If you're overhead LHR or LGW, it would only be 5 miles away or thereabouts.... Does that mean it's the nearest suitable in such an event??
Maybe an airport 100 miles away would be more suitable to allow descent, planning, briefing, etc...
FWIW a colleague of mine had pretty much that happen...ETOPS Twin (early days of ETOPS) engine failure at TOD for main base but, as it happened, also directly overhead a very suitable and available airport XXX. My mate carried on to base....at the subsequent board of enquiry the authority did ask some very pointed questions but eventually accepted that continuing the descent to destination, rather than diving into the alternate was fair enough......However he was of the opinion that if the failure had happened before TOD the authorities might well have expected him to divert, and that they wouldn't have looked very kindly if he'd used a excessively lengthy decision making process to make home base even more suitable...
