You want POD's that create anarchy in England. There was a degree of expansion of Welsh culture at times, an example being the re-Welshification of eastern Flintshire in Norman times, and the expansion of Welsh into the area round Shrewsbury in the early modern period, but nothing like what you need here.
The marcher lordships in the Norman period are a possibility. It was during this period that Welsh solidified its hold in Archenfield in Herefordshire, and Maelor Saesneg, Deeside past Chester and Wrexham were definitely included in Wales. The Domesday Book shows that Cheshire, Herefordshire, Gloucs and Shropshire all had some Welsh landowners and peasants, this trilingual situation in the Marcher lordships is perhaps your best bet. If, say, a marcher lord of Chester or Shrewsbury, marries into a powerful Welsh family (and I think you need to ATL-up a spectacularly strong Welsh family) then we could have a King of Wales and the Marches, ruling from a city on the English side of the border.
Welsh has always been spoken by some in Chester, though it is not normally transmitted generation to generation. If we can turn Chester Welsh, and that is the capital of *WATM, I can envision a Wales that includes Southern Lancashire, Cheshire and Shropshire. The Welsh culture would become dominant in multilingual areas. If it can also get hold of Herefordshire and Gloucs, the kingdom has a chance of lasting, though it would be English-speaking on its Eastern border.