Can you file accounts and corporation tax for clients for who you don't undertake the bookkeeping for through Quickbooks for a one of charge?

QuickBooks doesn't support a standalone filing tool or a pay-per-return option for outside books. You cannot process a single yearly return there without an active subscription managing that client's records.
 

In order to use QuickBooks' compliance and tax tools to file accounts and Corporation Tax, the client's file must live within the QuickBooks ecosystem via QuickBooks Online Accountant linked to a client subscription like QuickBooks Online or QuickBooks Ledger.

If you want to learn how to prepare and submit annual accounts reports with Workpapers and Pro Tax in QuickBooks Online Accountant, take a look at this article: Prepare and submit annual accounts using Workpapers and Pro Tax in QuickBooks Online Accountant.

Let us know if you have further questions or concerns.