It's interesting to note that some very different time restrictions have been added for the test fares from likes of Tonbridge and Tunbridge Wells to London. The morning restrictions applying from 06:00 to 09:00, i.e. half an hour earlier:
Purchase flexible single/return tickets & Travelcards – in 3 clicks, at face value with no fees. Every British rail fare shown, railcard discounts included.
www.brfares.com
Obviously this isn't definitive as the West Midlands PAYG test fares were set up with no evening restrictions, and subsequently had these added after the DfT insisted that the resultant
revenue loss (sorry, "overcrowding") would be unacceptable.