In the current game I am playing as England and early on I captured cities, and then eliminated Kongo from the game. This was a very long time from when museums could be built so neither of those cities already had them. So it is now late in the game and I am well on my way to a culture victory and I wanted to build another Archaeological Museum in the once Kongolese capital. Upon completion this museum only has slots for 3 relics, not 6 as the British ability provides. All indicators in the game show these cities are "British". I didn't have them ceded to me, in fact there was no-one to cede them as Kongo was eliminated. None of the other AI are displeased with me for warmongering or occupying another's city yet apparently they are not.