Monitoring mortality rates provides essential insights into public health, revealing the hidden impacts of diseases, enabling the identification of trends, allocating resources, and guiding effective policy decisions. Here, we explore premature mortality in dogs and cats across the United Kingdom (UK); we leverage a novel methodology that examines data from 28,562 deceased dogs and 24,460 deceased cats. We utilise an automated large language model-based ICD-11 syndromic classifier and reveal the impact of behavioural conditions on premature euthanasia in dogs, particularly as the leading cause of years of lost life in ages one to six. Examining cephalic index variations, our study highlights a 36\% increase in premature mortality risk among Brachycephalic dog breeds, underscoring critical animal welfare concerns compared to their Mesaticephalic counterparts. Importantly, our study establishes a significant correlation between premature mortality and socioeconomic status, highlighting the persisting inequalities within the UK's veterinary field. Lower index of multiple deprivation (IMD) scores are significantly associated with reduced premature mortality in both cats (0.52, p<0.001) and dogs (0.53, p<0.001). This research underscores the necessity of examining the socioeconomic disparities affecting animal health outcomes. By addressing these inequities, we can better safeguard the well-being of our companion animals.