Of course kent has nothing to do with Thorpe, but how it shows how much planning goes back and forth, local area, environment, other companies around the area. as for the build, its not something that would take a few months
4 months, possibly. merlin can whip up a new building easy, because there's nothing in it's way. An existing building tends to have infrastructure running through them supporting other buildings, services, and then you also have local residents and restrictions on movement over areas of the park. So moving staff to go via another route may not be possible. Then you have other events happening during the close season. Knocking it down and starting from scratch would be a huge feat, and personally I can't ever see it happening anytime soon, definitely when theres little wrong with the building.
As for green lit quickly, each area, country has it's own restriction, some can deal with builds quicker than others