A quote from P33 - Feasibility Considerations:
“If for any reason Heathrow cannot be expanded, the
next best location is near to the current Luton airport.
This airport is on the right side of London for the rest of
the country, and can easily be linked with high quality
connections to London, the M1, the West Coast Mainline
and the East Coast Mainline. There are disadvantages:
the site is relatively hilly, and Stansted would have to close. In
addition, business individuals that have located near Heathrow
for access to the airport would need to relocate. For those
reasons Luton is second best, but deserves to be considered."
Oops - that seems sure to ruffle a few feathers!