Zed isn't an alternative pronunciation. It's the original (derived from the Greek, Zeta). Zee was adopted by lazy, feckless Americans because it rhymes and panders to the uneducated. In fact, the United States is the only country in which Zee is the standard. To assume it should be pronounced Zee is typical of an ego-centric American point of view, and hardly surprising given their stance on the Imperial system! (And no, I'm not British).
