I don't think the BBC generally produced matrix encoded masters, I think they only encoded at the point of transmission. The reason being that they didn't want to lock particular versions of the fast evolving Matrix H into recordings but could use whatever was the best variant available at the time of the broadcast.
Your likely correct @Soundfield. But I think perhaps a small number of BBC Concert Quads in H/HJ do exist.
Surely they have some of those encoded tapes in their own Tape Vault ?