Could you explain this a bit more fully FTLA?
To recap: for the journeys the OP requires ( from Wymondham and Attleborough to London ) , the cheaper (route Cambridge) tickets are not valid via Norwich although change-of-route excess is, naturally, a possibility.
The more expensive (route Norwich) tickets are valid via Norwich (and, of course, Cambridge as well if wanted.)
So the OP should not have any problem with the intended journey (on a via-Norwich ticket going via Cambridge for one of the legs.) .
AFAICS journey planners should show via-Cambridge routes for the via-Norwich ticket but not vice-versa.
..I don't think it's easy for them to program in that 'via Cambridge' tickets can sometimes also be used via other places which aren't Cambridge..
Are you suggesting the journey planner should be able to pre-empt a potential change-of-route excess? If so, that's quite an ask.