Hi Robert,
The two minutes was thought to apply when using validators at a station, but it turns out it’s ok, unless the validator has continuation exit applied (West Drayton doesn’t). If you use the same validator then you need to wait 30 seconds between touches, but if there are two validators you can use a different one to touch back in immediately.
As Alan alluded to, PAYG fares beyond zone 6 are designed as a convenience and to match the paper ticket pricing for anytime and off-peak return tickets. There are some big savings to be made if you step back a train at West Drayton, but the pain is that you have to use the slow trains.