I suspect this is because the UK has a data sharing agreement with the US, but when Brexit kicks in there is not a legal mechanism in place for easy transfer of data from the EU to the UK (which makes your life hard if you're a company based in Ireland and you need to send data to the UK.)
Are there limitations with using products/doing business outside the UK, from within the UK? As in, is there any authority that they could use to stop people from, willingly(?), using a foreign internet service? It seems that would require some pretty draconian internet policies.
Or maybe even just XOR it with a random key, put the result in the UK and put the key in the US. That way neither US nor UK has "personal data" -- independently each country just got a pile of white noise -- but the two countries together do have it, and they wouldn't be able to say who "really" has it.
This is about what's profitable and within legal policies of a country they are operating in.
Where data is stored is sometimes an issue of jurisdiction policy (laws).
Think of this as a separation between requirements (what) and implementation (how)