Under IATA rulings, 2.1.9.2 defines an open jaw stopover as follows:
- When one or more portions of travel are by surface transportation, the last point of arrival by air and the first point of departure by air on each surface break count together as one stopover if the time interval between arrival and departure of the air segments constitutes a stopover as defined in 2.1.9. [More than 24 hours].
Then it further clarifies:
- Where a specific routing or fare permits stopovers at named points, both points must permit stopovers as defined in the fare rule or by limitations.

In other words - you may come undone!!

I know with Star itineraries I was doing when AN was still around, what you are proposing was a no-no. The point of departure and point of arrival although considered as one combined stopover, are still counted individually under stopover rules - which does make sense as you have to count both Warsaw and London as stopover cities, but it's only one stopover.

Cheers.