Solar farms - 35 acres/5MW-approx. - £13m up front, return after 9-10 years but 25 year life span of panels and in a decade's time the money just keeps rolling in for doing zilch. To get the best feed-in rate though you've got to do the deal right now, no more time left to waste.
Running airfield - 10 more years, resurfacing, repairs, hassle, peppercorn rent always considering redevelopment of brownfield site, constant demand for new housing with incentives to build 'affordable housing'.
Quiet little airfield is lowest on list of options for freeholder.
Not good omens on this front - unless you can have both solar farm and keep the runway?