Since you haven't made Anglesey a province but it was one of the most important sacred places in Britain, might I suggest a wonder either for the island as a whole - Inis Mona - or some monument on the island, such as Bryn Celli Ddu (*"Briga Ceti Dubi" or something similar in Old Brythonic, if you assume the Welsh name means the high-place in the dark wood).
Anglesey was a very fertile land, so perhaps it should give population growth bonuses. It should also increase the loyalty of all Celtic factions to its owner, as historically when Suetonius Paulinus and Julius Agricola attacked it their intention was to break the back of British resistance by striking at their religious heart.
Bryn Celli Ddu (*Briga Cete Dube):
"Bedd Branwen", the alleged burial place of the Welsh heroine/ Celtic love-goddess Branwen or *Bronawinda could also be a possibility. It was originally a circle of stones at the mouth of the river Alaw (to do with Alaunos?) but today only one stone remains (and it's not very impressive :/) and dates to the mid-second millennium BC. Excavations carried out there discovered cremated remains underground, and that's how it was dated. The grave of the goddess of love is not a bad idea for a wonder if you ask me...