OTL Waterloo has a British force (at least a division, I thiink more) that took no part in the battle, as it was defending from a potential encircling move, and a Prussian Corps that fought at Wavre, so there's a cadre of troops that would remain to stiffen resolve, as well as entire Russian and Austrian armies descending on France.
It's also difficult, if not impossible to totally destroy both Prussian and Anglo-Durch forces. Difficult enough to do it to one, to be fair. But say that D'Elerons attack is entirely successful, Wellington is killed and his army chopped up by cavalry as it streams to the rear, the Guards, KGL and Rifles surrounded and destroyed in Hougamont and La Haye Sainte, (this is a massive stretch for me, btw - while D'Eleron was having initial success there remained infantry reserves, let alone the cavalry charge that actually did for him). If this happens, then the Prusian liaison officer (Muffling?) rides over to Gneisenau and tells him the 'English have been destroyed' - the Prussians turn around and brush past Grouchy, saving their army to attack again with the other continental powers. Britain mourns its loss of prestige, brings more veteran troops back from America, reactivates more ships and decides to write off Waterloo as yet another cock up in Belgium (a la Walcheren).