I think that what you require is possible, but a bit convoluted.
First you need to go to settings=>chart of accounts and add a unique inconspicuous character to the end of each of the income/expense accounts that are relevant. (In the example below I have used a semi-colon ;).
The relevant accounts in your case are Inventory - sales, Inventory - cost and Discount expense.
That done, create a Custom Report with the following settings (note the semi-colon in the account name contains parameter):
This is a sample of the output (the invoices in this case have items divided into 2 lines for tracking code purposes CGT/Non CGT so is reported correctly and not duplicated):
I hope that this is helpful. Cheers