This is another one of the not so clear rules/regs.
BS6891 specifically mentions compression joint and union joints must be in accessible areas to allow for tightening the joint to make gas tight. It does not specifically say threaded fitting cannot be used.
But, does a threaded fitting not need to be tightened to make gas tight?
I was taught that mechanical fittings cannot be used in inaccessible areas and that is what I have stuck by. I suppose it is open to interpretation, but I do not see how you can allow a screwed fitting but not compression.
As far as I am concerned the safe option is a soldered cap.
Maybe the reason it does not specifically say threaded fittings cannot be used is due to the Malleable Iron installs, but that is a different entity carried out under a different set of standards.