If things go differently, even a relatively late POD would suffice. Welsh identity is a fairly tenuous thing, really. In terms of material culture and subsistence, religion, politics and family patterns, the Welsh don't differ much from the English. What the identiy latched on to early is language. Now, Europeans can talk a good game about bloodlines and faith of our fathers, but language is really what national identity boiled down to. If you spoke the language at a certain point, you were in. That is why Wends and Obodrites are Germans now, but Schleswig Danes and Sorbs aren't.
If the great majority of Welsh spoke English by 1750, they would today be as 'Celtic' as many a Wiccan from Essex. Unfortunately, I don't know enough about Welsh settlement patterns and cultural developments to say how this could be achieved. Greater urbanisation, perhaps?