"Hong Kong, which doesn't just have serious land constraints"
have you ever been there??
Re-read what I said. Not JUST serious land constraints - ie, as with Japan, reclaimed land used because of a lack of available undeveloped + flat land in the city.
I'm not aware of any cases of whole airports (as opposed to runway extensions on established sites) being "voluntarily" located in offshore locations, when flat undeveloped land is available somewhere else, as it is in abundance at Stansted.
Of course there will be unhappy locals, but they are a minor force compared with maritime geology, and Stansted is still on the table, as is Gatters, as is LHR3, as is do nothing, all of these in preference to FBI.