22mph at entry corresponds with a brake rate setting of 0.4 m/s/s in ATO, which is what LU say they use in poor adhesion conditions. Are you suggesting this is what they use all the time on surface sections of Northern? If so I will try to find out why, because it contradicts the information provided a couple of years ago.
As regards time, the difference between stopping at 0.4 m/s/s and 0.8 m/s/s is about 7 seconds over the platform length, but if this applies all the way down from 40 mph it will be more. Against this is that a properly tuned ATO shouldn't need to "flare out" before the stop, which is what most human drivers do. This saves a few seconds.
Yes. The reason given is because of the likelihood of the VOBCs halting if the train goes into a slide, and the time then taken for the VOBCs to be re-entered into the system, which involves travelling at slow speed to the next inductive loop. In some cases the gentle brake rate will be all the way down from 50 mph, and in the future may be from 62 mph which is intended to be the maximum speed in the long-term on selected sections.
If you're referring to the human knocking the brake off before stopping, this is done to give a smooth stop, and shouldn't cost more than a second. A good ATO system should have this designed in anyway.
Not that the tunnel braking is necessarily better either. To give an example, at Elephant & Castle southbound there is a point on the tunnel wall where the tunnel changes shape, which many drivers used to use as a marker to start braking from 20 mph. Now ATO is in, the speed coming into that platform has reduced from 20 mph to 18 mph -- guess what, the ATO starts braking *earlier*, from a lower speed. However this is starting to digress from the usefulness of ATO on the mainline.
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Very basic question here: why do trains not have antilock brakes? Is it just, or mostly, that it would be too expensive due to the number of wheels needing to be monitored?
They do, but same as with cars it only mitigates, it doesn't fully remove the issue. It works by momentarily releasing brakes on individual wheels if the system detects the wheel has locked up. Naturally it follows that with brakes releasing all over the train, brake force is reduced, although the idea is that it's still more brake force than would be the case if the wheels remained locked up. Where the train eventually stops will depend on how bad the conditions are and whether the wheels start turning again when the system re-applies the brakes.
It's similar to driving a car on ice, if the conditions are bad enough it will slide -- and in the worst cases a train can slide for miles. The only real mitigation is anticipation and driving accordingly. Sand is not guaranteed to work either, and the problem with sand is that if you use it all the time then eventually it will run out.
), losing defensive driving policies will speed things up ALOT (note: I don't think they are a bad thing at all, but it's a different kettle of fish with ATO) to name a few. 
