A few things.
It is not law. It was a proposal.
That proposal specifically stated that it wouldn't be to Euro 6.
In fact, all the proposal was doing was making passing the basic emissions test, which is already part of the MOT as it stands today, a requirement. In other words, you would be required to meet the MOT limits of a 2002 onwards engine.
The test methods for emissions in a MOT and any of the Euro stages are very different.
If it were to become part of the IVA, then it wouldn't be applied retrospectively.