It'd be impossible to enforce in the first place and it's obvious to every power in Europe. The monarchists wanted a king, sure, but not a British king, and the revolutionaries didn't particularly want a king once the heads started rolling. Unlike the Hundred Years' War, there's no support on the mainland for a British king trying to take over France and the British army's not strong enough to take on the French army AND occupy all of France while the people would rally to either the French royalists or the revolutionaries. It'd just bleed Britain for no real gain and a whole lot of bad press in continental Europe. None of the other powers need to intervene since France alone could push out the British (at the start of the French Revolutionary Wars, Britain's army had fewer than 50,000 men while France fielded hundreds of thousands once they began mass conscription). But it might make future cooperation a bit more difficult.