On 24/8/39 UK Parliament passed the the Emergency Powers (Defence) Act. That gave Govt. rights that (IIRC in his WW2 History) Churchill noted as exceeding the (legitimate) powers of any totalitarian regime. Property, any private property, could be requisioned for (the sense is) Defence purposes. All normally: to be returned to the original owner when Defence purpose lapses...but, sometimes, Govt. imposed compulsory purchase, at (some kind of) pre-War, normal market, price.
For airfields, before you rush in to buy from (whatever MoD Estates is called today), you need to know whether a descendent of the original owner can still claim title. The surveys done in 1938-42 identified even more sites than were actually turned into airfields: Policy from mid-1938 was to build up a b....y Paralyser Force, to encircle, blockade and thus reduce Germany. After Anschluss, Auto industry ceased civil work 21/4/38, RAF Expansion Scheme L was funded 27/4, including 250 Halifax (10/38: 500), and the contracts placed by 11/38. Agency Factories were funded to build all this. I have 560 airfields built Sept,38-Sept,46, from 2,000 surveys: in ’44 £1Mn. (Civil Works, plus land requisitioning) for each triangle layout RAF Builds for War,HMSO,97,P113. Paddy fields using hardcore helpfully provided by the Luftwaffe were turned into airfields by 60,000 strong men, many volunteer Eire citizens. Done in fewer months than the years taken today to do a modest stretch of road.
1942 was the peak airfield building time, after FDR & WSC agreed Germany First and the Combined Bombing Offensive.