Not sure if this helps anyone, but Sky now appear to do a 31 day rolling contract for BT sport at 25 pcm for their existing customers ion their platform, which can be switched on or off on their website. This might be an option for those who just watch BT for the rugby 9 months a year, or for those who notice that we have a lot of games in a particular month that they can't get to in future- effectively a one off one month for 25.
Sky are also are bundling the two packages (Sky plus BT) for 40 pcm on an 18 month contract as you would expect. By comparison, watching regular BT Sport on Sky via BT themselves is a whopping 30 pcm with no contract, but if you ring and ask for a contract they seem to be offering 24 pcm on two year deals as standard, so a pound off Sky's rolling contract but you are tied to a contract. I know they also do bundle deals for their platform too.
Either way watching on Sky you get the BT channels but not extra, but the red button service does work (but cannot be recorded).
Not trying to sell anyone on a platform, but thought the rolling contract thing is something people with Sky already and no BT might wish to consider if they are not STH's with access codes to BT coverage.