Look at the maps in Heirs of Merlin and the Lion and the Lily. Look at how the Covenants are distributed.
In Normandy you have 4 Covenants along the coast of the English Channel (Fudarus, Confluensis, Dragons Rest, Spiders Palace) and another 2 within 25 miles of the coast (Exspectatio and Florum). Wheras Stonehenge has along the English Channel coast (and thats being generous as its East Anglia) is Nigrasaxa. And there are absolutely no covenantss on the south coast of England.
Now, to me, that points to some kind of terrible conflict where Stonehenge lost quite decisively. Combined with the Norman covenants hunger and struggle for vis, I could see the Normans having used either politcal warfare or outright violence (perhaps during the Schism War) to control the English Channel coastline and destroy or retard the foundation of any Stonehenge covenants along the coast. Given the ease of crossing the channel, perhaps the Normandy Magi consider the South coast of England their own personal Vis reserve and regularly harvest the vis there? (much to Blackthorns anger)
Maybe any Stonehenge covenants that would be too close to the Normandy ones were overran during the Norman invasion. The unity of the Normandy Tribunal versus the more freewheeling British Isle tribunals would suggest such a detente among the Norman covenants could easily agree to eliminate the Stonehenge Tribunals via mundane political means, freeing up easier access to vis and other resources and removing them from their preferred shipping lanes.