I think the real point ought to be a comparison of the applicable contemporary norms of behaviour. I have read Denis Smith's book "The Prisoners of Cabrera" and am halfway through Paul Chamberlain's "Hell Upon Water: Prisoners of War in Britain 1793-1815". I have yet to find in either book (one Canadian author, one British) any indication that Britain's treatment of prisoners was any worse - or any better - than that of the French, for example. Indeed, there are indications that British treatment of prisoners was infinitely better than that of the Spanish, by comparison. Not surprising, perhaps, when one realises the predilections of the guerrileros for sawing their captured opponents in half, boiling them in oil and otherwise mutilating them.
Morla was theoretically responsible for the early treatment of the prisoners who ended up at Cabrera and his letters to Dupont and others reveal some of the external pressures he was under from the population at large, whose desire seems to have been to slaughter the prisoners rather than imprison them. Although the Cabrera episode is a shameful one, I find it difficult to believe it was a deliberate policy of extermination from the beginning.
And I just don't understand how the British bogeyman can have been complicit in the treatment of French prisoners by the Spanish. Precisely what role could they have played in dictating standards of treatment of prisoners of war by another sovereign power - which had already demonstrated a complete inability to listen to or react positively with other powers unless it was seen expressly to be in their interests? The Spanish refused the offer of British ships to repatriate the French PoWs, as the Spanish authorities repudiated the Convention of Andujar signed by Castaņos and rejected the option of shipping the prisoners home. How is British complicity indicated?
I'm not sure I agree this is a red herring - but I would be interested to learn whether you have any actual evidence for your belief, or whether it is founded on 'facts' and assumptions not in evidence.
Cordially
Tim