You can do as you say although I don't know if they can force this if bot seats are still bookable as SAvers.

Alternatively, do a dummy booking and force it to give you Flex fares. It'll be more expensive (by a lot more than £13) and should come up during the booking process whether or not it qualifies.

However, I've just tried it and it doesn't appear to.

What you could do is call and ask about that specific itinerary, or book it and choose Bank Transfer as a payment option. Then you can call and if they say no just don't complete the payment and the booking will be cancelled.