Both were tribal, and both rejected the Roman institutions after 410.
I think I disagree there : both didn't rejected Roman institutions, but kept what they already had from them. The maintain of tribal identity in Roman Britain doesn't mean Britons rejected Romanisation, but that the creolisation in the province didn't affected social structures.
For instance, see Christianisation (which was a marker of Late Imperial romanisation) that wasn't rejected by Brittons.
The big separation would be, for me, between urban Britto-Romans that may have looked a lot more like Gallo-Romans, and rural Britto-Romans that had really different references while still romanized (by this, understand creolisation of roman imperial and late celtic cultures).
Another rupture could be the South, more similar to the continental situation, and the North and West, that had a different and more distant roman influence.
With some kind of Briton civilization rise earlier than even the Gauls in France, and this is pretty difficult to achieve. Maybe some greek colonies in Britain like Marseille in France?
Well, tried to propose something akin
there, but as you said, it would certianly have huge butterflies.
You had a Britton civilisation blossoming in pre-Roman Britain, which is called the culture of hillforts in English I think?
The easy way would be to have a more tied up Southern Britain/Northern Gaul relationship (An actual Halstatt period in Britain? Maybe trough partial move of population as in IIIrd century?) and earlier Roman presence (as in trade, not conquest itself) in Atlantic Europe.
If not an earlier contact with Rome (and that's going to be hard) is possible.
Maybe a slower conquest of Gaul, scaling on more years than just a decade. It would increase both ressources (from roman presence, influence, trade, clientelism) for Celto-Britton states, and need for more strong political ties.
Defeat before Cimbrii? Caesar's head serving as decoration in an Arverni temple? Everything that would delay a bit would be appreciated, but coupling it with an earlier interventionist in the west in the same time would be as well quite interesting (Carthage being crushed in the 1st or 2nd war?)
Your guess is as good as mine there.
As i already said, it probably was the Justinian Plague to wipe out the Welsh, maybe because they had different settlement patterns than the Saxons.
I'd be more careful about wholly separating Germans and Britto-Romans : it's likely that the line of Wessex comes from a mix between Brittons and Saxons as the name of their first kings hints.
Don't forget, furthermore, that there was a German presence in the South of Britain since the IIIrd century, what happened in Britain was clearly not a struggle between two "totally-non-touched-by-Romanity" peoples