Dont expand it - move it!
Luton is a sod to get to from most directions for anyone north of Watford because of the crap road connections, (from where I live Luton is 82 miles, Stansted is 85, it takes one and a half hours to get to Stansted and nearly 2 and half to Luton).
I know this raises the "new airport - NIMBY" issue", but the sale of the land which Luton sits on would make significant inroads into the investment required on an existing ex airbase site nearby.
Most of the jobs there would be portable, as would be the base operators.
OK, big scheme - but worthy of thought?
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It wasn't me.