Unfortunately, if you didn't have a separate mux, then you wouldn't be able to use the DVB-T2 modulations, which gives an additional 40% capacity. And, during some of HD tests over recent years, there were concerns that some existing receivers wouldn't be happy with mixed MPEG2 and H.264 on the same mux anyway.
On top of that, you'd probably end up having to use Dolby Digital for the surround sound, rather than any of the more efficient new codecs. And that in turn would mean that for Audio Description, you would either have to forgo it on the HD service, or broadcast an AD broadcaster mix, which wouldn't be terribly efficient.
Given that the HD service manages to provide audio (albeit with the current reservations regarding surround) with 320kbits of bandwidth, plus another 64 for AD, and the gain by using the HD mux is 40% extra capacity, I'd say that the use of a separate mux is far from inefficient.
The T2 mux gives, roughly, another 10Mbps of bandwidth. That far outweighs the potential saving of a few shared soundtracks, even if you could ensure that none of the installed base of boxes falls over when presented with a stream containing both types of video.