Fairly sure his lecture load is both light and limited to first years because a) he's busy with the ATLAS project at the LHC; b) a charismatic lecturer is a great way to keep first year students interested and committed to that difficult first year at uni and c) he spends a lot of time popularising science on shows like Wonders and whatever the current one is.
The guy does a decent job popularising science, but I don't think he would be the best person to take over Sky At Night. Chris Lintot and Lucie Green are doing a great job. Real scientists with real passion for their subject.
It doesn't always have to be a rock star. Sometimes the fact of their celebrity makes them slightly suspect - on the other hand, if I have a question about the sun, I can do some research, then ask Dr Lucie a question on Twitter and 90% of the time, she'll either answer it directly or point me towards a source for my answer. Try doing that with Cox or De Grasse-Tyson.