The VAT adjustment journals have been set up in such a way that you cannot amend box 6 / 7 without amending the actual VAT figures. it also seems odd that only the side (debit / credit) which the VAT code is on will be amended on the return.
I have to post a journal as a result of another issue - I am unable to report my cost of sales by using a purchase invoice as customers pay sales invoices net of the cost of the sale, however, I do not make a physical bank payment to pay off the purchase invoice. Thus my only choice is to raise sales invoices net of the cost of the sale and then post a journal to credit sales (increase the sales to full value in P+L), debit cost of sales (show the cost of the sale) and debit input VAT (reclaim the VAT on the cost element). Sales are zero rated but the cost is standard, therefore I don't want to post to output VAT but I do want to show the correct figure in box 6.
Another example would be if both the cost and the sale were zero rated. If I want to post a journal to increase box 6 and box 7, I should be able to debit purchases and credit sales without touching the VAT codes but yet still updating the VAT return boxes.
There should be an option to "click to include" or "click to exclude" from the VAT return under the VAT journal in order to allow the correct boxes to update.