The £2 million investment is not something that has actually happened. This news piece seems to be about what they plan to do over the next two years as opposed to any actual investment thats taking place. Infratil have already said that any capital investment will need to be justified. Traffic at Manston does not justify any investment.
Lubeck, although talked about as "under performing", it happens to be the only IAE airport that is actually growing in size. As Infratil have an option to sell this month and they want to raise capital, its an easy fix.
If they had an option to sell either Manston or Prestwick as they do with Lubeck, there is no doubt either or both would be sold.