But then what was the link between Druids and Anglesey? It may be that there is none but that it just happened to be the last place the Druids were driven to by the Romans' inexorable advance, Anglesey being opposite Dover and all. Or it may be, as the Megalithic Empire points out, that Anglesey is a handy centre for Britain and Ireland.
Druids were certainly drawn to islands -- or at any rate land/water interfaces -- so Anglesey might make for a goodish capital anyway. Certain islands (I can think of the Isle of Wight and Sheppey offhand) are unusual in having a long equidistant coastline adjacent to the mainland. Could be important in itself, could be a sign of artificial carving out.