Most of the Discovery Channels' new material is made in the US, not the UK. Most other channels are exactly the same, mostly repeats, and what new stuff there is made in the US. Sky 1 and Sky Living are about the only channels that do do new stuff that is UK produced. Yes, they do a lot of new imports too, but they have a feeling of being stronger than most other channels. Even Discovery's channels pretty much don't feel like they're on a different level from channels like Dave, or even the CBS channels.
Er what has UK versus UK content got to do with it? They can still be popular and strong offerings even though they're mostly American. Factual is an area that is lacking outside of pay TV and the big 5
Personally the documentaries section is the one genre I miss since I left pay TV and went Freeview and the fuss created when Discovery threatened to leave Sky last year shows I'm not alone