The UK would probably have mostly minor changes, they’d be a lot more like France but still very similar to our timeline because of their isolation.
For the Rhineland and Austria I see a way of how they could’ve developed:
Austria wasn’t very inhabited even when the Romans collapsed, so with a high percentage of Latin population they’d probably gain independence after the Lombard occupation and act as some sort of buffer state between the Italian states and the german ones, then they’d become a bunch of communes like the rest of Italy, I doubt they would be united and since the HRE was keeping alive the ‘big’ Italian states in the Middle Ages the other peninsular states would be even more disunited. The HRE would be significantly weaker as well, maybe not immediately but it would eventually fall to the Polish and French, Bohemia would act as a less authoritarian and weaker Austria.
The ‘Austrian’ communes would then eventually become part of Venice, which would be a much more powerful state with no Austrians to fight, and then, the Serene republic could easily unify Italy, maybe with some sort of pact with the Two Sicilies, but i can imagine them unifying at least northern Italy.
In the meantime, the Rhineland would suffer the same fate as real life, they’d be conquered by the Francs and then be part of Charlemagne’s empire, the only thing would be that they would remain a part of France.
Basically in this AU I see an incredibly strong France, there isn’t any Austria to stop them, the HRE is weak, the UK shares many traits with them and are maybe even seen as allies, the northern Italian states allow their influence and maybe even the annexation of Genoa and Piedmont.