There are indeed BoJ restrictions, but on super off peak returns only. Wonder why they have this restriction in the first place though.
Unfortunately you have fallen into a trap for the unwary - you need to click into the fare details to get the full story.
Walk-up tickets such as Super Off-Peak Returns permit break of journey unless otherwise specified in the restriction code.
Restriction codes JA, JB, JM and JN don't contain any restrictions on break of journey. They purely contain restrictions on departing from/arriving at London and Manchester.
Ticket types OPR and OPS (confusingly used for Super Off-Peaks) show in BR Fares as barring break of journey on the outward portion, but this is incorrect as break of journey isn't allowed or barred at a ticket level for walk-up tickets. It is instead defined using the restriction code.