I sent in a help request to Sky and got a phone call. I was told the only way to get an advance notification of the Direct Debit amount is to switch to paper billing (which costs extra). I was also given a postal address with which I can take the complaint further.
It's possible that Sky will say that you can access your bills online, so you can discover the amount to be debited there.
Those companies I deal with that use online billing with direct debit for variable amounts all send an e-mail when the bill is available to view online. Do Sky not do this?