Furmanek,
Manston is on a peninsula, the passengers can only travel there to/from the west, not from the north, south or east, alas to the west of Manston major airports already exist with available capacity and/or empty seats.
Should, at a time in the future, London need more capacity there might be a case where LGW goes to a dual runway operation, they've already considered more runway(s) at STN, Southend is developing, Cambridge is sitting there with an M11 link to/from London, Biggin Hill might 'lose' it's 'ticket sales' prohibition and then there is Birmingham with approval in place for a runway extension, a motorway complex, rail station and monorail already in situ and with train times from/to London set to decrease.
There is no need for an island airport and Manston stands very little chance of significant scheduled traffic, perhaps some IT charter work but Manston stands a better chance concentrating on maintenance, freight, parked up aircraft etc.