Cardiovascular magnetic resonance (CMR) has transitioned from a specialist tool to a pivotal, multiparametric modality shaping diagnosis, risk, and therapy across major cardiovascular diseases; however, guideline positioning remains inconsistent between the European Society of Cardiology (ESC) and the National Institute for Health and Care Excellence (NICE). We performed a structured synthesis of contemporary ESC and NICE guidance (2015–25), mapping recommendation strength, evidential certainty, and pathway sequencing across heart failure, cardiomyopathies, ischaemic, and valvular disease, anchored to a curated PubMed dataset for transparency and reproducibility. Thirteen ESC and five NICE documents met the inclusion. This was a structured guideline review with narrative policy synthesis. ESC integrates CMR early with graded, disease-specific roles for morphology, tissue characterization, stress perfusion, and flow quantification; NICE adopts a more conservative, problem-solving stance, typically after echocardiography or CT triage. Concordance is strongest in heart failure and valvular disease, whereas divergence persists in cardiomyopathies and myocarditis, and in chronic coronary syndromes, where CT-first pathways dominate in NICE. These differences alter diagnostic yield, time-to-aetiology, radiation exposure, and downstream allocation to revascularization, electrophysiology, genetic testing, and surveillance, with measurable implications for equity within capacity-constrained services. Methodological drivers include ESC’s emphasis on clinical utility once analytic validity is established vs. NICE’s explicit cost-effectiveness modelling and deliverability thresholds. Aligning recommendations around shared outcomes, rapid-update pathways, and commissioning levers would reduce unwarranted variation while preserving fiscal responsibility. This review defines where and why CMR’s value is realized or deferred in UK practice, and proposes pragmatic, evidence-linked strategies to harmonize guidance and accelerate mechanism-based care across populations, regions, and clinical acuity levels.