Gatwick I believe has a long term planning restriction that blocks a second runway there for many years
Ends in 2019. Chance of proposing, planning, approving and building a runway there before 2019. Nil. So surely it is now an irrelevance?
FDF - more one we've done on other threads, but considering the challenges of getting a 3rd runway at LHR, I'd rate the chances of getting a fourth within the next 20 years as somewhere below the nil chance mentioned above.
Now as for HKG, I supposed it used to be "part" of greater UK - there is also the small question of land reclamation, which ain't cheap, and wouldn't such a runway also need quite a substantial taxiway in order to make it widely enough space from the other 2?
Speaking of colonies, let's not forget MNI, re-opened in 2005 after the previous airport was covered in volcanic debris. Not strictly new in the capacity sense, but still very much a new runway, and as per above,at 553m full length for the kind of services operated.