I'm no employment law expert. but surely if he's sacked for being crap we can appoint who we want whenever we want? And the same if he resigned?
If he resigns he can claim constructive dismissal, if he's sacked he can't, which is why he'll have a compromise agreement, meaning you're fired but it's a nice way to do it & you feel like you aren't quite so hard done by, but it also means he'll be gagged for ever & a day & if he even mentions anything about why he got fired he'll have to pay back everything, is the usual terms of such agreements.