I'll share details to help with your question about restoring data, Steven.
QuickBooks Online UK and QuickBooks Self-Employed does not have a backup copy feature. Please know that you can no longer go back to QuickBooks Self-Employed once you switch to QBO.
This is the reason why you can no longer access or see your transactions in QBSE since you've already changed platforms.
If there are transactions that are not transferred over to QBO. You can import invoices and banking transactions to complete your records. Check out these articles for more details: