Over the years, we've re-fought several actual historical Napoleonic battles. These, as I'm sure you know, have the challenge of converting history to fit on a wargaming table with miniatures & rules. This 'what-if?' still has parameters but not as many. Both aspects are definitely fun.
So, in this "universe", the British were beaten at Waterloo, but the French were too disorganised/exhausted to mount an immediate & energetic pursuit. This gives the British time to rally & although they've shed their Dutch-Belgian allies, they're still formidable as they head for the Channel & possible evacuation.
The next game will follow the Prussian defeat & the subsequent pyrrhic victory we've already gamed. Blucher is, for a while, capable of no more. Wellington must. like Sir John Moore, halt the pursuers. So a sizeable force will confront the next enemy - a Corps under Davout (who must have taken the night express from Paris). So, roughly 13 infantry battalions + the usual for each side. More a set piece battle rather than the rear-guard actions fought so far.
Exactly when this will happen, I'm not sure. Scenario is written.
donald