How will they ensure they continue working or train replacements if their parent company no longer employs or pays them? What's to stop them leaving the country?
They are _not_ hosting UK government data in the US. They offer on-premise and self-managed cloud offerings, which are often used where data sovereignty is important. Additionally, customers often manage access control - including for Palantir employees - themselves, so it would not be a simple "pull the power plug" if the US/UK relationship went awry.
For what it's worth, a friend of mine is a lawyer in a well-known hedge fund and he gets access to their funds too (funds that would not otherwise be accessible without making a substantially larger investment I believe).