Amazon use DD+ for their HD streaming service; my Gen2 FireTV has a setting to select DD+ or DD audio on the HDMI output, so it can be disabled if your AV receiver objects to the additional DD+ information, which I believe should be discarded/ignored if the DD protocol has been implemented correctly (discuss!).
I do think it is a bit strange that Sky haven't adopted it given that DD+ actually needs lower bit rates (and smaller files) compared to the equivalent DD stream for the same audio quality and we all know how broadcasters love to keep those bitrates as low as possible to squeeze in more channels.
Box installed yesterday and first impressions are it’s fast and the voice control is really impressive which I normally care less about. The hassle of finding UHD programs is bit annoying but I read somewhere that Sky are likely to improve this in the near future. The Dolby Digital + content is a lot less than I was hoping for but overall I recon I should have upgraded ages ago.