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 these instances, (and in my experience it's rare for QNH to change rapidly enough to require such exchanges), the controller will often simply announce the new altimeter setting on frequency and say "all aircraft acknowledge by ident". It seems to work well.
I would be supportive of changing the the transition altitude to 18,000 ft and transition level to FL180 worldwide. In addition to clearing 99% of the planet's terrain, it also represents the 50% point for atmospheric pressure and seems like a natural place to delineate low altitude and high altitude flight. Also, raising the transition level/altitudes to these heights allows the change to come at a less workload intensive phase of flight in the flight deck, whereas the flaps are still being retracted during the changeover in places such as Amsterdam.