Problem I see with making it all dual carriageway is that you’ll get what we get on the A1; massive tailbacks with one lorry taking 15 miles to overtake another with a 0.001mph speed differential!
Then you have to make it a 3 lane motorway (much of the A1) or ban lorries from the overtaking lane at peak times (seems a fair bit of this on the North East A1).
As for average speed cameras we had them on the M62 for a while while the managed motorway was constructed. Everyone moaned about the 50mph limit but traffic did seem to flow better resulting in my run from Huddersfield to Castleford (25 miles) actually being faster at busy times.
Also there seems to be better traffic flow with the motorway management, I have not gone to complete standstill for months now.