I haven't seen any other threads in the past few years delve into a notion like thus, so bear with me.
With a POD no earlier than 400 AD, could an alt-Dalriada be established roughly in the same spot as OTL's Gwynedd or South Rheged (which I don't recall being any less habitable than OTL's Dalriada in terms of climate or farmland) that leads to a Gaelic colony in OTL Wales, while simultaneously seeing the Britons in the North (either north of the Humber or Hadrian's Wall, whichever is more plausible) hold out against Pictish, Anglo-Saxon and Norse incursions long enough to survive as a separate nation?
If so, which country would likely be swallowed up first (or at all)? How would British history end up being different with both lands effectively swapped so?