Why did Britain allow French intervention despite the Monroe Doctrine?

Why did Britain permit French intervention in Mexico when British sea power helped sustain the Monroe Doctrine?

British creditors benefited from French debt collection, while the Royal Navy retained the ability to interrupt French maritime support if British interests required it.

British creditors could benefit from French intervention because France bore the cost of collecting debts. Ships crossing the Atlantic could replenish at either end, limiting the need for intermediate coaling stations. The Royal Navy nevertheless retained the ability to disrupt the maritime support on which French operations depended. British tolerance therefore depended on British interests.