This seems supremely complex! So the journey planners and their somewhat bizarre selection of allowed trains must be right then. This gives allowed trains between London and Reading of:
1612 to Reading (stopper)
1618 to Bedwyn (fast to Reading)
1622 to Oxford (calling Slough)
1627 to Oxford (semi-fast stopper)
1642 to Reading (stopper)
1657 to Reading (stopper)
1706 to Westbury (calls Twyford)
1718 to Oxford (calls Maidenhead, Twyford)
1725 to Banbury (stopper)
1736 to Oxford (calls Maidenhead, Twyford)
1750 to Worcester (calls Maidenhead)
1757 to Reading (stopper)
and similar services until the last barred train leaves at 1915. The choice of services seems a bit bizarre to me - a mixture of a few slow services that will probably be busy with commuters and some other services.
Is there any particular historical reason for this somewhat strange set-up? Does any other region have it? And couldn't the restrictions be incorporated into the NFM, or does this apply to any ticket of these types that could possibly be routed out via Paddington?
The restrictions have been in place for many years, certainly since my mate was at Uni in Reading in the early 1990's.
IIRC, the reasoning behind these restrictions is that the off peak tickets, such as a Cheap Day Return, were banned for the long distance services due to overcrowding. Travellers on the HST's to Bristol, Plymouth, Cardiff etc, who pay very high fares to travel at peak times, would not be able to board the trains as they would be full of Reading travellers on discounted tickets!
There were similar restrictions on CDR's and Supersavers on all the old IC routes. I used to have to pick up a little pocket booklet that showed the restrictions, for example, from Liverpool Street to Norwich if I wanted to travel back via East Anglia!