If it was still in army service I'd expect itto have military plates and not be on DVLA database.

Not taxed and on public road = offence. While driving it other than to a pre arranged test would be an offence not sure one is committed if it's parked.

Is it insured?

ownvehicle.askmid.com/

If its not being used is it a danger to anyone, appears to be taxed and presumably insured so whats the problem, so the owner has forgotten to MOT it or is (more than likely) waiting for a break in the weather to do some work on it to get through the test is that something to grass someone up for.

Sorry if that seems harsh but some people have to suffer from the actions of anti social bullies, drug dealers and assorted nasties and the old bill couldn't give a stuff unless the victim happens to be an untouchable/minority in which case a media sensation hate crime leading to gongs and promotion is in the offing and they can't get there quickly enough.

I have no doubt a fixed penalty (kerching) will arrive promptly on the owners doorstep if you report it, but that's only providing more revenue for the ever diminishing service we receive from our various aloof and distant apparatchiks or more funds for our national unleaders to fritter or give away.

Yes the MOT site is extremely useful for additional checking out of a potential purchase.