There is nowhere in SE England to build a 'world hub' if such a thing exists other than the new UAE airport which currently no one uses .
There is no point at all having two airports however theyare divided since in SE England any land journey takes a ridculously long time or is ridiculously expensive if by train.
If Boris island was built the train fare from the IT Valley between Guildford and reading M3/A40 corridors would be more expensive than the airfare to places like Paris Amsterdam etc etc.
Of course LHR isn't ideal but it is the only realistic choice -the reason it is there in the first place is because its on the only bit of flat ground anywhere near London and with usual lack of foresight (perhaps pardonable at the time) was not to leave all the area north of LHR as open land rather than just half of it and developed the airport with that area shown from the start as provision for a third parallel runway.
Now its on to HS2 - 2050 anyone?