It was in fact Wales that was the only lght of christianity during the dark ages. The Welsh church was apastolic in foundation - being founded by St Ilid (Joseph of Arimathea) soon after the crucification.
Indeed, the Cymru were the very first christian nation - one of the earliest churches in the west being founded at either Llandaff or Llanbedr y Mynydd (by King Lleirwg).
The Roman church was founded by a Briton as you have mentioned (Emperor Constantine being the half British son of Empress Helen - who was a British princess)
If only the Welsh were aware of their history!!!