Hello, @AntM.
First, make sure that your personal account is added to your QuickBooks Self-Employed (QBSE). Then, prepare an Excel CSV file for the expenses that were paid using that account. Once completed, import the file to QBSE.
To import the file, select the Import transactions tab from the Add transaction drop-down and follow the onscreen instructions to complete the process.
To help account and categorise your transactions accordingly, I recommend consulting with a professional account. This way, they'll be able to guide you more closely and help you record the transfer of amounts between your business and personal accounts.
Also, check out this article for additional references about the SA103F Categories.
Mention my name in the comment section below if you have any other questions. I'm always here to help. Have a good day!