1. We aren't yet exempt
2. I suspect that emission regulations are somewhere near the bottom of the pile of things that need sorting out once (if) art. 50 is triggered
3. I'm not sure that there is a desire to relax emissions regulations. What government would want to say "We are allowing dirtier engines again"?
4. By the time we have freed ourself (if at all) from them, the 66 and 70 will be a massively outdated design that won't be offered any more
This was discussed at length recently along with other impacts of Brexit on the railway
In general I agree, the only plausible exception IMHO would be if a Hybrid or dual powered 25KV and diesel locomotive was desired, perhaps based on the proven 66s.
If most of the mileage was to be on electric power, then it could be reasonably argued that emissions on diesel would be of little consequence.
Such a locomotive would not really be a 66 though even if the diesel parts were so derived.
Another possibility would be a modified 66 permanently coupled to a modern purpose built AC electric locomotive. The pair would be permanently coupled and modified so as to be driven as one, in effect it would be an articulated loco, not a coupled pair.
Under diesel or electric power the traction motors in BOTH units would be powered thereby making full use of the weight of EACH unit for adhesion and reducing the chance of slipping.
The idea has considerable merit IMHO, but firstly it certainly wont happen just yet, and secondly it wont be a 66 when built, even if one end started out as a class 66.