Pacocha and the difference between survival and victory

Was the Battle of Pacocha really a British loss if Huáscar withdrew and neither side was sunk?

Yes in operational terms, because the Royal Navy failed to sink or contain Huáscar and therefore did not achieve the objective it had set.

Both sides remained afloat, and Huáscar withdrew after nightfall, which can make the engagement look tactically indecisive. Sources disagree on whether the British projectiles count as armour-piercing, semi-armour-piercing, or neither, so that ammunition claim remains bracketed. The clearer measure is the mission assigned to the Royal Navy force. The British intended to blockade, contain, or destroy the Peruvian ironclad, and Huáscar escaped. Damage inflicted without achieving that purpose did not satisfy a navy whose institutional standard demanded decisive success. Battle assessment must therefore distinguish tactical exchanges from operational objectives, which is why the Royal Navy treated the outcome as a lesson.