The VHF / UHF issue is irrelevant.
The hours of operation of UK military aerodromes
should be publicly available in the UK Mil AIP. But that has now been hidden behind some dumb MoD sick-urity firewall....
So how are people expected to know the
official hours of operation?
Anyway, some military aerodromes may have RAF Flying Club activity and/or Air Cadet flying activity outside their normal hours of operation. If these aerodromes (e.g. Benson) have an ATZ, then you may not enter it without permission. If the resident Flying Club is operating, even though they may be making normal RT calls in the circuit, they
cannot approve ATZ entry.
Aerodromes such as Abingdon and Little Rissington, neither of which have ATZs, may have Air Cadet gliding activity. The only avoidance you are required to observe is the same as for any normal civil gliding site - although in the not-too-distant past Little Rissington seemed to think it was Heathrow at times...