While many of us may gasp at 25 million passengers passing through Luton by 2025, I suppose it is inevitable the airport will be planning for these sort of numbers by then!
To achieve the 25 million mark, it "only" requires a yearly growth of 6.5% a year until 2025.
6.5% growth per annum sounds very modest/tame, compared to growth in past recent years! Below is what this looks like on an annual basis.
I suppose once the airport reaches 20 million by around 2020/21(ish), growth cannot grind to a standstill... Also Luton and Stansted by that time will be the only places in the London area where any significant air travel growth can take place.
It all just boils down to how the "little airport on top of the hill" wishes to plan and play this game. It also depends on how deep the airports pockets are, and how long the arms are, on the person reaching into those pockets!
To increase the terminal capacity by a further five million, isn't going to require a whole lot more space and remember this is Luton who does things on the cheap!
One idea could be to literally fill the whole CTA with an expanded terminal and shft reliance of dropping passengers off outside the terminal to a new area and shift the strain on to this new mass-transit system that will be running by then and expanded to cope... The terminal could be expanded south and a new pier could be built where the exisiting multi-storey is, and looking at how cr@p the construction of that is, there is no way that will still be standing by 2025!
Also they could put in an apron on the otherside of the taxiway, with a Gatwick Airport style bridge over the taxiway, from the current pier.