S.O.E.
... if it's managed well.
Ah, yes, the vital words. Given that British govts have spent 25 years telling BAA to do what it thinks is right - and then cussing them for making the wrong choice! As well as so weighting the charges at make them make more money out of the shopping mall ... I can't see any change happening. The suggestions you make are all very sensible and therein lies the problem!
The standard British SOP (Hold An Enquiry - Then Do Nothing)
is a viable option. The current recession and changes long since entrained by Euro hubs and the ME carriers will create enormous changes across the next 25 years.
So, I agree that LHR3 will soak up capacity and, as I have repeated before, if there was a regulation to cap the holding time to a maximum of ten minutes under normal operating conditons (as might be expected from the planned timetable and not yet affected by weather or closed runway) THEN the capacity is (probably) almost already accounted for! Capping holding times is a real win for the green lobby and pax - but I bet no one mentions it.