Bad Bear:
Look at the volume of airspace that is the Scottish TMA (inc EDI and GLA) and think how much of it is actually needed. Then think how aircraft that do not get clearance through have to take large detours you will see why CAS is a problem.
EDI and GLA virtually never deny clearances through their CAS unless you want to plough right through their SIDs/STARs at 90kts in IMC
If one were to plot the actual tracks that are flown it would represent a small percentage of the volume of airspace.
Then again a fair chunk of the Scottish TMA is Class E allowing you to blunder through VFR without even speaking to anyone - and that's a bit which is extensively and intensively used by Glasgow inbounds!
Airspace reserved for cross runways that are seldom used
But those bits of airspace are also used for the jet SIDs off EDI 06 and GLA 05 so not much would get released even if they shut the cross runways.
Airspace down to the surface 10 or more miles from the airfield
A legacy, I suspect, of the fact that the GLA and PWK CTRs used to be one big CTR called the Scottish CTR i.e. there used to be more CAS, not less (maybe you didn't know that).
Airspace below RVA
The EDI example has been raised by 1000ft in the last 2-3 years. The GLA chunk is Class E so hardly an imposition.
SIDs that reflect climb gradients of aeroplanes long since consigned to museums etc.
Both EDI and GLA have some quite demanding SID climb gradients and they were increased when the airspace changes were implemented 2-3 years ago.
NS