Nice idea...
I think this is a good idea in principle, but it relies on all road users being alert and competent (which, I think, is where it begins to go wrong...).
You are alert, approaching a traffic-light controlled cross roads at 30mph in a 40mph limit intending to go straight ahead.
Then someone pulls out into your path, turning left through their red signal.
Nice to know you were in the right, but you are still in a mess...
Looking at the number of injuries involving emergency vehicles in this country, treating red lights as "Give Way" (as they are entitled to) and then colliding with innocent motorists, I fear the situation would be much, much, worse if less alert drivers are allowed to do it?
And the emergency vehicle driver stands a good chance of getting prosecuted for driving without due care (for other road users) too.
I suggest that one of the reasons we have often the lowest fatality rate in the world is because we reduce the risks motorists are allowed to take (there will always be someone who knows better and so takes that risk, but Darwin takes care of them sooner or later...).
It *might* cut congestion in a few places, but no more than widespread use of filter arrows in appropriate places (where it is a single carriageway and the car in front of you is waiting to go straight ahead but you want to turn left, this rule does not help you).
You can be 100% confident that within a few weeks of this being allowed, there would be examples of people being killed or seriously injured - TBH not remotely worth it.