1. STORY: offering the idea that the Blue Stones were moved one at a time over several centuries, each stone, each time representing a chief cultural figure who had passed on (a sort of pre- Amergain type leader and lineage and their stone was transported to Stonehenge via Glastonbury. 
2. ROUTE: 
Glastonbury as the prequel destination before Stonehenge. This is important in offering and testing ideas about why the journey was made. Glastonbury remains our oldest site of faith and ritual on current evidence, which I present for testing that it is the strongest route that makes sense both on the ground, through trade links, possible beacon usage, and in terms of continuous cultural importance as a ‘faith/ritual’ centre. 
The sailing route options are interesting. Lavernock to Brean Down is shortest (in this era). That’s why it’s the proposed location of the often-projected Severn barrage, and why it’s the place where Marconi did his radio trials. Detail below.
 I work with clustered sites of neolithic and bronze age. It’s possible to find evidence of continuity of land usage over several millennia of settlement, and trade and socio-faith connection can be easier to hypothesise. I call them ‘celtic ancestral landscapes’ (until I find a better term).
 I hypothesise that the journey to bring the stone(s) down off the ridge route from Pembrokeshire and Carmarthenshire runs into the Vale of Glamorgan (as per early sections of The Celtic Way). The Vale take precedence for me because it has over 50 sites with Arthurian toponyms (not necessarily of the possible 5th century war leader, but a cult of Arthur, e.g. the ‘Head of the Bear’, (Pen +Head Arth = Bear in Welsh) as in Penarth, the bear-head shaped promontory poking out into the Severn. More detail here. 
Since 2018 I’ve intermittently been working on a possible summer solstice sunset alignment between Glastonbury Tor, Brean Down, Steepholm and Lavernock, hence my interest in this route for crossing. Launch pads and steppingstones crossing to a coastal landing point or water course for Glastonbury seems worth exploring. Sully Island looks good to me. More detail here. 
These as well as sea conditions are critical with the crossing. No one wants to lose the sacred stone form (or forms) of one’s Amergain figure. On a flat sea, 5 hours of tide, doing a guided drift across the Severn at this point would take how long? Well, 16 miles at 5 knots tidal pace? The option of leapfrogging from Sully to Steepholm to Brean gives rest-points or escape stops if the conditions worsen or offer planned island stopping points – steppingstones - and then on into the Levels. 
Finally, for something this important I’d expect maybe poling on and off beaching points, some roping for approaches and departure for the Bluestone(s) barge or raft.