Yes I've seen plenty of PJMs and have mounted quite a few for guys whose attitude has been "sod the MOD, I'm retired and I'll wear my medals where I want". I very much doubt if Her Majesty would be offended, but of course I may be wrong there.
They are a bugger to mount, though, as the ribbon is quarter of an inch wider than the usual British width, and therefore they have to be squeezed on to standard brooch bars with a bit of judicious overlapping!