You are in the UK I believe?
Any aircraft flying in the UK (apart from a few unpowered things) has a registration. If civil and not intended for a specific G-xxxx registration, it may fly under a "B-conditions number", which is allocated by a CAA approved flight test organisation. So, it'll display (and you'll write in your logbook) G-nn-nn (first two digits belong to the B-conditions organisation, second two digits are allocated by that organisation).
So far as I know, all UK military aircraft under flight test carry the usual two letters + 3 numbers registration that any other military aeroplane will carry.
One Slovak company that I had an association with for a few years have a transferrable registration - OK-DYNA which appears several times in my logbook, each time for a different aeroplane. That was a little confusing !
At the end of the day, one can always find something to write in the logbook so long as there is air under the tyres.
G