One person's "controlling their own engines" is another "spewing nitrous oxides, carbon monoxide, and other pollutants into the air, giving cancer to neighbors and destroying the atmosphere". We tried the "don't regulate" path and it ended in a multitude of disasters.
I don't think "enormous costs" is a strong argument here. The annual MOT is usually under £50, and that includes a fairly comprehensive roadworthiness check and emissions check. A stronger argument would be that a car owner could change some settings on their car specially to pass the emissions check, and then set them back to dirty afterwards each year.
An MOT is a cost to every driver every year - significant in money and in time - plus the cost of running the infrastructure to provide the testing. All so that a few people can tune engines? No thanks.
