If you intend to break your journey with this ticket then in my view the best way of going about it is to ask the member of staff onboard (i.e. the guard) whether this is permitted. They are, 99 times out of 100, going to tell you that break of journey is permitted. If they do so, you are at liberty to break your journey entirely legitimately.
In the extremely unlikely event of them insisting on the restriction, you can simply offer to pay the relevant difference to the cheapest valid fare permitting break of journey, to the member of staff at the barriers or (excess) ticket office at your intermediate break of journey station. In all likelihood they, too, will tell you nothing is due and that you are entitled to break your journey.
I don't really know why WMT still maintain the break of journey restrictions which LM brought in. It makes no sense from an overall perspective (i.e. these fares don't really undercut any intermediate journeys' fares) and are simply not known about by staff, let alone enforced, in my experience.