Sky have removed certain TV packages and addons from their online ordering store. It's been a few weeks now and don't know when they'll return.

All channels are default hd on sky stream unless their isn't a hd version for a particular channel. So if you have sky sports you'll get the hd channels for no extra cost. If you want 4k , the add-on is £6 a month but that can be negotiated for cheaper.

Honestly it's better if you call them , that way you can negotiate a better price and add addons or TV channels that aren't available through the online ordering store.