And don't forget that the net realisable value of Lyneham is b&gger all! The land was requisitioned under Defence (Emergency Power) Regulations and because of this, the original owners/descendents have to be offered the site back at the unimproved value. [The 1939 purchase price was c ₤12,000]
Additionally, there are areas of contamination that the MOD would have to either pay to remediate or transfer the risk to the new owners, in return for a substantial reduction in the disposal price (which would be low, anyway). In this case, cheaper to retain.
Barracks would definitely need to be improved to SLAM standard; amongst other reasons this is because the Adult Learning Inspectorate would hit the roof if trainees (Phase 2/3) were put in the remaining blocks. The last major upgrade took place in 1995, when 2 new en-suite blocks were completed. Most other accommodation blocks would probably be demolished.
In terms of technical accommodation, I suspect hangars would remain – the UKMAMS hangar had a deluge fire-restraint system and all have 3-phase power (some with other voltages) fed to them.