My point about the UK was not some 'Little England' nonsense - it was to highlight that an 18000 ft TA, whilst appearing attractive to airline traffic, will increase RT and ATC workload for traffic below, climbing through or descending through 18000ft. Every time someone comes on frequency, they will need to confirm and readback the relevant QNH - and if there's a significant QNH change all traffic on the frequency will need to acknowledge receipt. That will apply to everyone, not just those cruising at a constant level.
The UK has nothing like the same level of sectorisation as the US and a high TA could lead to a non-stop babble of QNH exchange on busy frequencies. This would be less likely if the TA was down at 6000ft.
In the UK, traffic is allowed to fly IMC outside CAS - without even needing to talk to anyone or to squawk. If the UK retains its archaic 'regional pressure setting' system based on forecast, not actual atmospheric pressure, separation between traffic outside and beneath CAS and that flying inside CAS could be significantly reduced.
With increasing pressure to reduce separation minima, anything which might lead to altimeter setting errors must be examined with considerable care.
The economic argument is phooey - as is most of the global warmists' envirofundamental greenwash. An airliner still has to get from the ramp to cruising level; where and when it changes altimeter setting doesn't alter this one iota.