You would have thought the airport would have sorted a special area for red zone passengers from the beginning! Still better late than not at all.
It isn't that simple. Heathrow uses their terminals primarily based on airline groupings, not destinations. E.g. BA uses terminal 5, Star Alliance uses terminal 2. The airlines share ground handling and servicing facilities (much of it contracted out).
So if an Air India (Star Alliance) flight from India has to go to the 'red zone' terminal rather than the usual terminal (2) then arrangements have to be made for the ground handling to be provided at a different location to normal. Given the criticality of safety in aviation, that isn't something you can just 'do'. Method statements would have to be re-written and approved, staff need to be trained, and all the necessary equipment put in place. That takes time - and a lot of money - to organise.
There's also the issue that each aircraft usually makes multiple trips per day. So, for example, a BA aircraft might arrive from India, and have its next scheduled flight to Singapore. If the arrival has to be handled at the 'red zone' terminal, but the departure has to be from the usual ('green') T5 then in addition to all the usual servicing, fueling and maintenance, the aircraft itself would need to be moved (most likely towed) from one terminal to another. I.e. more planning, training and expense.
None of the issues are unsolvable, but neither can they be implemented quickly. And overarching everything will be the assessment of risk - it is quite possible that an extra death is more likely to happen as a result of making hasty changes to airport operations than for there to be an extra death from Covid.
But it does make sense to separate passengers from different destinations, and that is why it is happening.
