Basra's oil access outweighed Hormuz's geography

Why use Basra as a support base instead of the more strategically placed Strait of Hormuz?

Basra provided oil and politically accessible facilities, whereas Hormuz was controlled by independent Iran.

The question separates a strategically attractive position from one Britain could actually use. Hormuz offered control over the Gulf approaches, but it lay under independent Iranian authority in the political setting described here. Basra connected to oil and support facilities in Iraq, where Britain exercised much greater leverage. Basing decisions depend on sovereignty and access as much as on the geometry of a map.