Henry Stern, a british missionary, was held hostage by The Emperor of Ethiopia among with his assistant and the people sent to negotiate for his release.

So as a result the British invaded Ethiopia, besieged the capital and rescued them all.

That's the most famous example of the british invading another country in order to protect their citizens abroad.

Without additional information - especially the year or at least general geographic area that it occurred in, there's not much we can do to help with any information. So, with that said...you sure you don't have any more information at all?