What to do with places like Bedford and Milton Keynes where there's both a period and a day return is an interesting question. I suspect in both these cases sales of the period returns are absolutely tiny, and so the singles would be based on the day returns or very, very close to them. There are relatively few flows where you get both, and they are almost all journeys of about 50 miles, where day trips are by far the most common thing people do. The more common case is that journeys under 50 miles only have a day return, and journeys over 50 miles only have a period return.
Period and day off peak returns are widely available for many shorter journeys to tourist destinations in the south east (to places such as Oxford, not just the coastal resorts). At the moment visitors staying for more then one day pay a premium for the off peak period returns. If the mooted changes "reward" this comparatively small group at the expense of the much larger numbers of day trippers, who pick and choose when they travel in line with the weather forecast, then local tourist boards and businesses may not be too chuffed.