The Tracked Rapier was originally not intended for the British Army, being a special order from the Shah of Iran. The latter ordered a mobile version of the British Rapier SAM, which used to be used as fixed AA position then. British engineers decided to use the same M548 chassis as the Chaparral's. But the Shah was overthrown by the Iranian Revolution before the vehicles could be delivered, so it was the British Army that finally acquired them.
8 missiles are mounted on the Tracked Rapier, with a search radar. It has both more missiles than the Chaparral and more accurate. Yet, it has less range than the later American variant, and must keep tracking its target after firing (making it vulnerable to SEAD attack), while the Chaparral's missiles, being heat-seeker, are fire & forget and invisible to SEAD.