@OliverBarrett, my previous investigations into using OneDrive in the UK was that all of Microsoft's servers are based in UK data centres. I haven't waded through their personal data documentation since 2018 when GDPR came into force and haven't investigated where the data centres are backed up to.
As per my comments in the MacBook Pro M1 Pro: questions - #15 by harvest discussion, I don't believe there is anything to prevent business use as per Apple's (in this case UK) legal notice Legal - iCloud - Apple.
Europe and the US have changed their privacy arrangements frequently over the last few years and each time they do, legal challenges put them in doubt again. The UK will no doubt come up with some form of flawed post Brexit changes to GDPR in the not too distant future, just to ensure we have something else to deal with instead of focussing on running our businesses. However, any business storing data about European citizens must be GDPR compliant.
Personally I think it is all a bit crazy. I can't imagine how much data Apple have about me having used and purchased Macs for over 30-years, having worked for an Apple reseller, being the purchaser of Apple equipment for our business and with a wife and 2 children who all have at least 2 Apple devices each. The idea that someone couldn't put my details on iCloud seems laughable. However, legally this all pales into insignificance!
All the best