Manston has the possibilities of being a London airport only currently if it services flights that don't need to really go to LHR, LGW and STN. What I mean by that is flights that don't have any competition- or shouldn't have- flights to some Asian countries and African countries, South American etc.
I, for once, agree with DeeDave, Manston is not a London airport, when it comes to European flights, and really shouldn't be sold as one. It does however have the possibility to compete as a small regional airport- but aircraft and routes have to be right. If EUjet had got her ATRs like the plan was certain routes would have proven to be more of a success. An airline the likes of Aer Arann could succeed at MSE- and Esperia could succeed if they get the smaller BAE146 for shorthaul and then use their MD83s for holiday flights- AGP, MJV, IBZ in the summer, for example, and GVA, etc in the Winter. But all of these routes would need to be sold with the Tour Operators and for Esperia not to go LCC- that market is soaked in the UK market, and not from MSE.