My thinking is that cars don't need to have ABS, it's an option a lot of cars come fitted with, so what's the problem?
However I can see the thinking along the lines of, the car is designed to have ABS, so you disabling it modifying the car, thus possibly causing the car not to be as safe as it could be?
Cars fitted with ABS must have it fully operational in order for it to pass the MOT. With it disabled the car would fail the MOT. And based on those grounds it would be considered un-roadworthy.
The only way I know to circumvent this technicality in terms of "modifying", is to completely remove the whole ABS system.