A 99 year lease isn't going to work. There's too much risk around technology changes in the long-term - investors will want to see a lot more of a return on investment in the next 20 years.
Let's assume it takes until 2030 before a new terminal is fully built and somebody turns up to unveil a plaque. The 99 year lease would expire in 2129, or 106 years from now. If we go back 106 years from today, we're talking the middle of WW1. The whole green movement only began to be taken remotely seriously about 40 years ago, and is now significantly influencing Govt policy. Furthermore, terminals at major airports seem to have a lifespan of max 50 years - so functional changes would probably require Luton T2 to be demolished and a new building erected - so the concessionaire won't recoup any money from the latter half of their lease
Banks and many investors won't lend to coal-fired power plants in Europe any more... there's just too much long-term ESG reputational risk on this for commercial banks, pension funds and their ilk.
I'm sure Luton council will try all sorts of things on this... but I just can't see this working without private sector investors getting proper equity ownership of some of this... or HM Treasury giving their backing