>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 :-)

As we don't know what they worshipped it's hard to say what the stones represent. They could have been war trophies. They could have been family totems from a tribe that moved. They could have been a wedding present from a Preselli based chief on the marriage of his daughter to a Salisbury chief's son - Stonehenge having been built purely as a place to hold a significant wedding which put an end to generations of war between the two groups.

Any theory is a valid one unless it involves reptiles.