Its a simple case of too much pollution above damaging levels, the gov't is being fined on a yearly basis (at their agreement). Then we have the coroners report for a child that died in 2012, setting a precident. The next step is to work out who to claim against for pollution related deaths (estimated 36,000/year in the UK).
The gov't doesn't like holding back commerce, so you can blame these namby pamby people with pre-existing conditions who expect the atmostphere to be breathable.
https://www.rac.co.uk/drive/advice/e...ons-standards/
The V90 will be EU 2 or 3, with combined limits for NOx and UBHC, it just won't have been tested for the seperate tighter limits. It would prohibitively costly to get that done, its not a stick-an-exhaust-probe-and-check-the-idle test.
The only conversion which will be approved easily is a BEV conversion, but I don't know if they are allowing it specifically. They should but its a big investment, probably cheaper to buy a new BEV. Hopefully we wil have a cheap conversion kit in the future.