But surely concerning the arrivals hall debacle they could use different terminals for green countries and red/amber countries.
That's been suggested, it may be an option, but it's not as straightforward a process as some seem to think.
Apologies if this ends up TL;DR, and please don't shoot the messenger but:
Different airlines usually use different subcontractors/teams to handle things like baggage offload, passenger disembarkation, aircraft cleaning, aircraft towing - chuck all "red" flights into one terminal and you'll need all airlines to have teams their, and all the associated kit that airline and it's specific aircraft type needs, such as specific catering loading equipment, aircraft towbars/tugs etc...That's one of the reasons why historically airlines have tended to group their flights into one or maybe two Terminals at multi terminal airports, so that their support team(s) can be concentrated under one or at maximum two roofs rather than scattered across several thousand acres of real estate.
There's also the issue that if you have "red" and "green" terminals what do you do if an aircraft arrives from a "red" destination but is then scheduled out to a green "destination"?
In the real world you can't lock an aircraft into a "green only or red only schedule, you need to switch them around for umpteeen reasons e.g. because they break, need routine maintenance. A gate switch within a terminal is relatively if an airline is fixed at one terminal but if you have to go to "red/green" terminals do you have to start towing countless aircraft across airport to switch terminals or do you bus passengers across the airport..
Short version of the above is running red/amber/green only terminals isn't as simple as has been portrayed by some (certainly one or two MPs) in the press.
Segregation within a large terminal would be easier to do.