In common with all nuclear power stations , Dungeness has a two mile radius exclusion zone up to a height of 2000ft. Lydd already comes mighty close to that on it's southern boundary. So as I see it the only possible runway extension is to the North East back towards the built up area.
Hence there is a noise/environmental problem. Why Shepway is short of information on this is puzzling, one can only speculate. (costs money).
The existing reactor (one only operational) is programmed to be used for another ten to fifteen years, so any new build is not relevant, unless it gets any closer to Lydd Airport. (sorry, Ashford International Airport)
Also another constraint to the SW of Lydd is the Danger Area around firing ranges.
Returning to Manston
I see this morning that there is still "an overwhelming demand" at Seguro.