The southbound equivalent Super Off-Peak ticket has a restriction 1K of 'not valid into London Terminals after 0429 and before 1117' (with variations for certain London stations). Although there is no service from Dundee to London to arrive before 1117, a similar ticket and restrictions exist for a number of other stations for ECML journeys to London. Does a similar principle apply, that one could use the ticket at any time of the morning as long as one breaks the journey before London, and does not actually arrive in London until after 1117?
I suspect this would be trickier to justify if using a London-bound service that is scheduled to arrive into Kings Cross before 11:17. For one thing, because of the way the electronic versions of the restriction codes are implemented, an 'arrival' restriction is applied to both the station where you get off a train as well as the final destination of the train. Therefore, you wouldn't be able to buy a Super Off-Peak ticket in conjunction with an itinerary which shows your intended trains. So you wouldn't have the contractual backing of an itinerary.
Moreover, the Train Manager may claim that they have no way of knowing whether or not you'll actually get off by the last stop before London. Obviously that's not how it's supposed to work, but you have to consider the fact that they'll have their train in mind as "peak". They may be sceptical of anyone who justifies "circumventing" the restriction by saying they plan to break their journey.
You also have to read the wording of each restriction code carefully. 1K is subtly different to how you have summarised it, as it actually says:
Not valid for arrivals into:
- London Terminals (except as shown below) after 04:29 and before 11:17;
I think it would be tricky to argue that trains such as the Flying Scotsman don't fall into the definition of "arrivals into London Terminals ... before 11:17".
The only thing that really goes in your favour is that some of the intermediate stations north of London, such as Stevenage, are listed separately with different timings. But I think the intention of the restriction is clear as it says "except as shown below". In other words, unless the station you plan to alight at is listed (Peterborough, for example, isn't), you have to apply the London Terminals restriction.