if you disable the seat on a Gnat or Hunter, you won't get CAA approval.
re the seats in JPs, they are more "relaxed" - as much as the CAA can be relaxed when matters concern ex-mil jets in civi hands.
not sure about other types like vampires........
one thing to bear in mind re ejection seats - stats show that military pilots have a far better rate of successful ejection than civi pilots. To the extent I believe that if you are a civi pilot, the seat is equally likely to kill you as save your life.
when i was flying a JP5 - with live seats - it would have had to be a situation which pretty much guaranteed death or serious injury before i would have used the seat. so serious structural failure? fire coming from the engine into the cockpit? loss of control from a spin and less than 5000 ft on the altimeter? ok - pull the handle.
but say you have an engine failure and you can recover to an airfield but wont land squarely on the runway? and the surface looks otherwise smooth?
the "not black and white"....and the price of fish question becomes real when you can't recover to an airfield. but you can see the mother of all open fields .....and it is mid summer...no rain for a few weeks...and the ground is hard. we know what the CAA advise....we know what the instructors advise (and in these days of litigation...they are not going to go against the advice of the CAA unless they want to be on the receiving end of a lawsuit.....), but if you fly these things, it comes down to pilot choice at the end of the day.