You heard me. Put Constantinople under the control of the Kingdom of England or the United Kingdom.
Anytime with a POD after 1453 but before 1914 (no LoN-esque mandate cop out).
Queen Elizabeth I's alliance with the Ottomans leads, after a few generations, to intermarriage between their respective royalty.
That would most likely result in Ottoman British Isles rather than English Constantinople. Though it would be interesting to have Elizabeth as Sultana of the Ottoman Empire.
and second one side or the other (of the marriage) would have to convert, which was not considered acceptable at this time.
Err...how? Besides the religious issue raised above, there's simple geography:an entire (sub)continent lies between the two realms. One country could not be governed from another in this case.From there both royal houses of England and OE join together to create House of Osmanli-Tudor. As the result of this England becomes Islamified while the Ottoman realms become overall Anglified. Yes this whole empire was at first dominated from London (due to eventual maritime dominance in the end), but short after that the French got to strong in Europe and began to threaten the British Isles, the capital of the whole Osmanli-Tudor realm eventually moved to Constantinople.(and now to run as fast as you can to the highest hills and peaks before the Welsh-bot came, QUICK !)
not the conversion of the ones who would sit on the thrones -- their siblings and-or cousins would convert and marry into the other dynasty.
I know that- but at that stage people were not so pragmatic as to accept it.