Hi,
SA103F contains a built in balance sheet for completion. You are free to include your version as an attachment.
SAI is a self contained summary of your accounts which you complete as part of your tax return. It provides a comprehensive summary of your trading and profit and loss account or your income and expenditure account, and your balance sheet. Also, it shows what adjustments you have made to arrive at your profit for tax purposes. You can see guidance here:
Information from your accounts 2023 (HS229)
If you intend not to have accounts prepared, the notes to the self employment (full) pages provide guidance to help you calculate your business profit, the adjustments you may need to make for tax purposes and how to fill in the SAI boxes. You can see information here:
Self-employment (full) notes
You must keep records of all your business transactions.
Thank you.