The DVSA book DRIVING THE ESSENTIAL SKILLS states
"Avoid braking so hard that you lock any of the wheels"
If you lock the wheels - you still keep moving forwards - If you lock the wheels and the ABS is activated - you still don't stop any quicker
ABS just provides a small amount of steering control that you wouldnt have had otherwise.
If you get used to stopping so harshly that the ABS activates, then if you have a car without it, the danger is, you lose total control of the vehicle
ABS DOES NOT MAKE YOU STOP QUICKER IT JUST ALLOWS A SMALL AMOUNT OF STEERING CONTROL