The expansion debate needs to be seen in the context of UK transport policy, or lack of it. I'm afraid Britain is very bad at successful forward planning, mainly I think because it is very crowded - particularly in the south-east. The fact is that all our major airports are overcrowded, but by good (short-term) management, cope with huge demand. The same applies to the London underground, the roads network and railways.
As long as people want/need to travel, facilities should be made available for them to do so. I will not address the climate change hysteria, as that brings in a whole lot of new questions and unconsidered possibilities.
Heathrow is overcrowded, as are Gatwick and Stansted. Yet there is still more demand for air services to/from London, to the extent that Southend has picked up a chunk of the market despite its location and limitations.
It makes sense to build new infrastructure to cope with the demand, and that can only happen at existing airports. I'm sure Luton will get its cap increased eventually, but the current T2 proposal looks like putting several quarts into a pint pot. Unfortunately, local government parochialism is preventing Luton from building new infrastructure in the obvious place - south of the runway.
I would hope that common sense prevails, and that sensible expansion can be approved. Aircraft have become much quieter since the first One-Elevens and 737-200s arrived in the late sixties and that trend continues with the neos. Surely growth can be accompanied by noise abatement planning requirements - for example, if you can't operate neos, you can't come in. And surely some of the noise nuisance caused by large corporate aircraft can be "re-allocated" to passenger airliners.