Like I say, BBC One could get away with using just 1 if they wanted, but '1' on it's own is not strong enough for Sky 1, but I'm not saying they cannot use it or they won't.
If I was running Sky1, I'd want the 'Sky' to be as much as part of the brand as the 1. The 'Sky', as much, if not more important here.
I'm not sure you're right. Sky are also using the standardised channel logo in promos, which is corporate. The '1' also uses their own font, so is unique in that respect.
It's certainly a bold move to concentrate on the '1' for the idents, as they'll need to develop more of them to enhance the idea of a character.
Fortunately, for Sky, they have deep pockets and this is an ident which can be used across their whole group.