I mean if you view colonialism as an emergent system brought about by material circumstance and economic interest, not all that much would change except Welsh (Well, Brythonic) would be the major world language.
The UK would still be a European Island uniquely positioned to disproportionately prosper from the colonialism era.
With Bretons owning England *And* Wales, it may even be more stable and positioned to unite the other celts on the Island. With less home issues, it might be more powerful.
If you're suggesting the anglo-saxons are rebuffed, you've also got to contend with Britannia being the last surviving Roman province and how that would impact their identity and imperial ambitions.
Potentially we'd see a lot more interference in the Mediterranean and closer ties with the Byzantines during the early through late middle ages, and the occasional British Emperor trying to reclaim western rome.
I suspect that wouldn't go well, though maybe the Byzantines might fare a little better under those circumstances unless they end up being dragged into Britannian quagmires trying to seize France and so on.
But when you get to the colonial era, not much changes except the UK is more united and has less domestic turbulence.
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The other scenarios are that Wessex falls to the Great Heathen Army or some such, in which case you've basically got the same as above, but now with Scandinavia as the hub of the British Empire, again potentially strengthening it, especially given the assimilation of the celts the vikings were a little better at than the anglo-saxons. Ultimately though I suspect that centre of power would move to the British Isles, albeit Scandinavianized.
Alternatively, Wessex simply fails to unite the Heptarchy. *This* is the one that dramatically shifts power away from the UK and toward competitors. The ramifications are enormous.
