The trains run at turn up and go frequencies but the fact is the walk-up fares are expensive so people book in advance and are restricted to one train.
I wonder if passengers are happier to pay off-peak walk up fares on the Birmingham corridor than on the more northern routes? Is Birmingham to London Any Permitted Off-Peak Return at £44.30 too unreasonable? Manchester to London is £70 so might be more likely to force people onto Advances from £11.50, as the savings are greater, even though there isn't that much difference in the SVR cost per mile.