FWIW: the British kept a force of 50,000 regulars at home (more British than Wellington had ISTR!), and Wellington had a very hard time getting reinforcements. IRL he had another division or two (?) en route from Britain.
The British also kept a force in the Med under Bentinck, and a portion of that (a Corps?) was also moving by sea to reinforce him, as indeed was a Portugeuse force.
If Wellington can hold at Antwerp, and the Dutch don't fall apart he might be able to strike out as Napoleon moves off.
Also, Napoleon has advanced the conscription, calling out next years class. Between Waterloo and crossing the Rhine he can expect significant reinforcement.