What is far more safe to assume is that it is a comparison between their current h.264 encoding setup vs. whatever h.265 coded they would use in practice.
I don't find that more safe to assume because if they were comparing any HEVC encoder to a non-reference H.264 encoder, i.e. a newer, higher quality H.264 encoder then their claims of 50% reduction in bitrate would not be possible.
Nobody in the encoder community has posted any sorts of encoder results with such a reduction in bitrate. What has BBC done that is so incredibly special to achieve what nobody else has?
I find it much more likely that they are comparing to a reference H.264 encoder as that would at least make their results possible, conformable, and repeatable in the first place.
Also, any HEVC encoder implementation produces results so close to a reference encoder that you might as well consider them all reference at this point. I've tested several myself. But H.264 on the other hand, reference vs. the best is quite a large gap.