Apologies for late reply.
You are precisely correct. In the case of a margin-scheme trader selling goods at a loss, there is no ‘loss relief’ to claim, such as when the profit on item A can be offset against a loss on item B. The loss on item B produces a ‘nil’ VAT liability. However, the global accounting scheme losses on an item are automatically offset against profits on items. Thus losses and profits are offset together in the period. Also note, with the global accounting scheme, all purchases made in the period are included, even if those goods are not actually sold in the same period.