just have the Brits declare they're taking them over. Portugal, in the 1800's, was never in any position to stop them. Do it between 1807 and 1813. Tell P it's to protect shipping lanes (just like they did with Dutch south africa), that it's the price P has to pay to get the motherland (Portugal) back, since Britain was the de facto occupiers/rulers there.
Britain took whatever it wanted from ashes of the Napoleonic wars, so simply give them a reason to want Portuguese African possessions.