sh650:
Class E is used almost 'E'verywhere in the US outside the busy terminal areas
Indeed. Which is why I'm surprised that the proposal to turn the Glasgow CTA - just such a "busy terminal area" from Class E to Class D seems to have disappeared into the ether.
It's also worth reminding ourselves what the UK air traffic rules say controllers must do when they see unidentified radar contacts in the vicinity of the IFR traffic they're controlling in Class E airspace:
Pass traffic information unless the controller’s primary function of sequencing and separating IFR flights is likely to be compromised.
So if it's really busy and you're controlling a string of arrivals you don't even bother telling the pilot about the unidentified return in your 12 o'clock
If a pilot requests avoiding action it shall be provided to the extent determined by the controller.
So even if it looks like there's a conflict, you wait for the pilot to request avoiding action
Give avoiding action if radar derived or other information indicates that an aircraft is lost or has experienced a radio failure.
How would you tell that from a primary-only contact which doesn't have to be speaking to anyone?
On top of that I'd wager that if you asked professional pilots inbound to Glasgow what class of airspace they flew through and what the rules were for it, half of them would be amazed to hear it could have unknown non-radio non-transponding VFR traffic in it, flying just below the cloudbase in vis as low as 1500 metres.
NS