It's probably seen as unnecessary as ATC give an instruction to descend to altitude giving the QNH, so you change to the QNH once vacating the current FL or vacating the last FL ATC wanted you to report passing.
Oh, is it really done like this in the UK? In this case there may be aircraft in the same space, at the same level, that fly on 1013 in level flight and some that fly at some totally different QNH while descending. How do you achieve vertical separation in this case? The whole idea of the QNE/QNH story is that all aircraft below certain level fly on QNH, and all above a certain level use QNE.