Thank you for getting back to us and providing us with more details of the concern that you’re having in handling your expenses, @Baked-in-cley.
Yes, you can when entering a transaction or reviewing a transaction imported from the bank/an imported CSV allocate a percentage to personal and percentage to business. If you click on the line of the transaction in the banking section in QuickBooks Self Employed and you will see a split option and with that you can allocate the percentage/amount how you wish and then add the transaction once complete.
I recommend that you consult with an accountant to help you choose the right SA103F categories when claiming allowable expenses for your mortgage and utilities. They are the ones who can advise you on which one is best suited to your transactions.
You can refer to the following article for more information about the SA103F deduction categories you can claim as allowable expenses in QBSE: About SA103F Categories.
See these articles for more information about adding your income and expense transactions in QBSE:
Also, check the following articles to help you get started with QBSE: Help articles.
Please let me know in the Reply area below if you have any further questions on managing your expenses or anything else. I'm always willing to assist. Have a nice day and be safe!