Yeah okay, as a UK citizens you may get troubles if you don't pay their fees. But you may be able to explain them that you don't have the docs any longer and they should just get the company closed. It can't be that difficult.
Once they smell money, they don't let go. I believe all these offshore jurisdictions are facing big problems now that it's no longer enough just to register a company offshore. And now they're trying to hold on to the remaining customers they have.
AFAIK, not required by law but often required in the T&C you sign with the CSP or required if the company has bank accounts or other regulated relationships which may necessitate KYC prior to offboarding.
While it's not impossible that they have a legitimate need for the documents, based on what you've shared, it unfortunately sounds like they might be taking you for all the money that they can.