Soldato
It would be a raft if anything, several trees tied together. The main argument against a boat or raft is how they loaded the stone onto a floating platform.
A crane and a dry dock would have been handy. I suppose they could use the tides and just roll the stone up a ramp onto the raft. That may give them six hours or so to get afloat.
Poling it along the coast without getting too far from shore and beaching at night would be likely avoiding storms it would take at least a month. Coming down the river Dee And whatever tributaries and marshland there was on the Welsh borders would be my guess then aiming for the Severn, Avon and or the Wye aiming for Wiltshire and the finish line.
Some mighty Druid magic there, he must have been a wise and clever leader.
