I've been looking into an idea for a timeline. The Idea is to keep Wales an Independent nation throughout its history. I've been looking for a PoD for a while but I can't quite find one. I want the PoD to be during the reign of Gruffydd ap Llywelyn, and between the years of 1039 and 1063. I feel somewhere in that timespan lay the best opportunity for an everlasting independent Wales.
Your biggest problem here is Welsh ... politics, for lack of a better word. Firstly, if one of the princes conquered other parts of Wales, he tended to split the land among his sons. That's got to stop. Also, the family/clan/feud based society was really unhelpful. Get your hands on a copy of Heather Rose Jones's "Welsh History 101B" (a song), with lines like:
"My brother's wife's fourth cousin's foster son Gruffudd is best for the job [prince]... What matter that Rhys is the old Prince's nephew, he's exiled to Ireland and cannot return" ('cause the narrator keeps having Rhys's boats burnt...)
Ireland had some of the same problems, but Wales had it worse, IMO.
Welsh society would have to change dramatically to have the internal cohesion needed to stave off conquering by a foreign power.