Is 18Mbps referring to the lowest I could expect before the distance from the cabinet takes effect? Or is that suggesting the overall lowest I should expect, regardless of distance, is 18Mbps? I would imagine the former.
The two ranges already take account of your distance (strictly, other factors influence speeds too, such as thickness and material).
You can expect your speed to be somewhere in the A range if your line is in decent shape - no dodgy joints, no extra branches of cable left attached, no insulation worn away etc.
You can expect the B range to apply if there are faults in the wiring.
ISPs will decide which range to use based on your installation choice; go for an engineer installation, and they should quote range A speeds ... and jump into action if reality falls well below these higher figures. Go for a self-install, and they'll use range B instead.
Whichever range is chosen, your actual speed is more likely to start out at the higher end of the range (or above it) if you are an early subscriber on the cabinet. It is likely to drop as time goes by, and more subscribers get added.