There is no tibet. The one signed with British was Qing dynasty. PRC is the successor of Qing dynasty.Not True.
British India signed agreements with Tibet.
India inherited those agreements being successor state to British India after attaining Independence from the British in 1947.
So Indian agreements are still with Tibet.
Just because PRC China invaded and occupied Tibet on October 7, 1950 does not make it legitimate.
China also still occupies Indian land in Ladakh but India still claims it and is part of official map of India.