Ex mil aircraft fly on a Permit to Fly basis. Therefore they cannot fly for Hire and Reward, so you cannot hire one, and you cannot be instructed in one unless you own some or all of it.
Sort of true, but not quite
Depending on the aircraft's owners wishes / agreement, you can fly their aircraft and pay the "costs" - under the terms of AIC60(W128)/2006 - without buying a share.
You never strictly get instructed on the aircraft, since as stated they are permit to fly. I am a JP "instructor", yet have no instructor rating... We essentially do a "conversion".... although with the 4 colour board briefs, soprtie profiles and write ups you would be hard pressed to not see it as "instruction"
As above, Swords (link above) offer the ability to be instructed up to (but not including) solo without being an owner. You do need to get the relevant "training exemption" from the CAA in order to log the hours... (and despite all the above, then as P U/T !)