Zaphod Beeblebrox’s full title was “The President of the Imperial Galactic Government”. The Imperial is anachronistic. The current Emperor is almost dead and has been for many millennia. This is due to on his dying breath, He was put into suspended animation - much to His Imperial aggravation. Since that time all known descendants have died so that there is no hope of sussession. The upshot of this is that all legislative power in the galaxy has passed down the rung a few steps and is now seen to reside within the galactic senate and a duly elected President. In fact, it resides in no such place.
(First cut from memory. If anyone has access to the original, feel free to correct the wording. I believe this is probably from the first episode of the second radio series, if that helps.)