Because if not, then losing against Napoleon would do it.
I don't know about Napoleon. The Napoleonic war basicly caused the British domination over the oceans. I would say avoid the Napoleonic wars or the French revolution and make certain that the French, Dutch and Spanish navy could still rival the British.
Around the American revolution most European countries were quite wary of Britain and this only changed because during the French revolution and the Napoleonic wars France suddenly was more dangerous than Britain again.
My suggestion would be something like avoid the French revolution (maybe create a constitutional monarchy in France). Another war between Britain and France takes place (they happened quite regularly, so seems almost unavoidable). The Netherlands* and Spain join France and their combined navy manages to defeat the British navy. The USA sees their chance and join to. End result Britain loses their Indonesian and Malaysian colonies to the Netherlands, Canada to the USA, various Carribean islands to France and Spain, British Africa to Franceand the Netherlands and part of India to France (and possibly the Netherlands).
* Dutch political problems had been solved somehow and they decide to invest in their navy again after the terrible result of the fourth Anglo-Dutch wars.