Yes the system is a bit dumb. Even though the advance fares will only be quota-controlled on the East Coast leg of the journey, the booking engine doesn't know that. If any of the trains on your itinerary are marked as reservable (e.g. the Peterborough to Cambridge leg), it has to wait until reservations open for those trains to be sure that the advance fares aren't quota-controlled on those trains.
If you go to a booking office with a knowledgeable clerk it might be possible to buy the ticket, as they should know that a reservation is only required on the East Coast leg of the journey. It probably depends on what TIS they use. If you wanted to try that I'd be very interested to hear how you get on...