you may have forgotten that the GMB has approval from the V. & A. to take photographs of items in the glass gallery, for posting on the Board, with the proviso that all such images are watermarked and to include acknowledgment to the V.& A. - similar to the Kew arrangement. This was a facility I agreed with them several years back, and I'm not aware this has been withdrawn - I have mentioned this previously on the Board.
Of course, if their reply appears to overlook this, let me know and I'll take it up with them again.
Would imagine you're correct, and that the 'no photography' rule applies to materials other than glass - paintings and textiles especially.