This study analyses the presence of the Sustainable Development Goals (SDGs) in the educational legislation and curricular materials of Spain and the United Kingdom, comparing their regulatory frameworks and evaluating the frequency of SDGs in textbook activities. A total of 555 activities from two secondary education geography textbooks were analysed using a non-experimental, descriptive-correlational, and cross-sectional design. The results show statistically significant differences between the countries regarding the incorporation of SDGs, with a higher prevalence in the Spanish textbook (66.0%) compared to the British one (41.9%), χ²(1) = 30.34, p < 0.001, φ = 0.234. An analysis of the co-occurrence structure among goals—conducted using the Jaccard coefficient and tested against a null model with fixed marginals and 1000 permutations—identifies three significant associations, corresponding to a social equity cluster, an environmental cluster, and a production-related cluster. The main finding reveals an inverse relationship: Reduced Inequalities (SDG 10) and Climate Action (SDG 13) coincide in only 4 activities, whereas 24.90 would be expected by chance (p = 0.002), and this pattern is replicated independently in both textbooks. The study concludes that the SDGs are incorporated in a fragmented manner, with a systematic dissociation between the social and environmental axes that transcends the two curricular contexts analysed.