Hello Dan,
Thanks for contacting us here on Community.
It is not possible to complete the Self Employed self assessment through QuickBooks Online, we do have a QuickBooks Self Employed product which includes all the categories for this which will then help with completing the self assessment by providing you with the figures generated by the transactions/expenses posted into QuickBooks Self Employed. This information can then be copied and pasted into the self assessment form from QuickBooks.
Please let us know if there is anything else which we can help with in relation to your QuickBooks account.