This is now available for a £5 pm premium over SD i.e. SS 1 & 2 HD for £27 pm. Only SD available when I started in July. I don't know if it was publicised so just in case anyone else missed it.

I've just activated it - 5 quid extra a month.

Interestingly according to the stats from my router it seems to be using 12mbit streams, as opposed to the 8mbit BT Sport ones. Picture quality is pretty good...