Just as with every "EU regulation" or "directive", the emission levels are embodied into UK law, either directly or by way of acts empowering ministers to make regulations without them being individually voted on by Parliament. So there would need to be another Act of Parliament or amended UK regulations to change the emission level rules. Somehow I doubt if this would be a high priority for any post-Brexit government, on the "it's not broke, why fix it" principle.
And class 66s - why, when the railborne coal traffic is in its death throes, would people order more general purpose freight locos. There'll be enough spare freight power soon enough, alas. Apart from that, I understand the actual model of diesel engine fitted to the 66s is now out of production, largely because it doesn't comply with US or Canadian emission regulations either, which was its home market