>If they subsequently decided to migrate to Salisbury, they may have felt compelled to bring their ancestors with them.<

Could be, Steve, though another theory is that we have a priesthood from Preseli who have relocated to Salisbury Plain and brought with them some or all their existing place of worship. That's not unusual, and we have examples of both religious and secular buildings (or things) being moved around from place to place - perhaps out of a sense of respect for those buildings and things or a need to maintain an element of religious and social continuity.*

* Why a new priesthood on Salisbury Plain? History gives us examples of the missionary factor, perhaps a splinter religion setting up elsewhere, or perhaps even the original place of worship becoming 'unclean' in some way - all interesting stuff :-)