The idea that a sinking of one or both of the new Carriers would generate a "War" is laughable at best.
Absolutely it would be a genuine "act of war" but that does not equate to "War" itself.
As the Man. asked....who and how would you invade the perp's turf?
Lay out the assets available to the British Military to accomplish such a feat....remembering if they can sink a Carrier....then sending some Amphib ships and some Frigates and the odd Destroyer to the bottom would be well within their means too.
Allies in the retaliation.....like the American Navy showing up in the Falklands or the Suez Canal maybe?