It's taken a while, but I've just noticed that HD versions of the vast majority of Sky Cinema On Demand movies are now available on my V6 boxes. We used to have these a few years ago but in recent times the only HD On Demand movies were the newest to Sky Cinema. Now they're back.
There are a few still missing (for example, I only see SD versions of Amistad and An Affair To Remember) but everything else seems to be in both an SD and HD version.