Atypical haemolytic uraemic syndrome (aHUS) is a rare kidney disease characterized by thrombotic microangiopathy. This study presents the first analysis of UK patients enrolled in the Global aHUS Registry, focusing on patient characteristics and disease natural history prior to treatment initiation (n = 172; 74 paediatric, 98 adult). Mean age at first aHUS manifestation was 23.6 years overall (4.9 years for paediatric patients, 37.8 years for adults). Additional thrombotic microangiopathy events occurred in 57.0% of patients between initial clinical suspicion and registry enrolment. Potential precipitating factors were recorded in 14.0% of patients. Of 115 patients at active sites, 90.4% had genetic data recorded, with 73.8% undergoing "complete" genetic testing (results entered for C3, CD46, CFH, CFB and CFI, as a minimum). Of those with genetic data available, 52.9% had an identified pathogenic variant. Gastrointestinal involvement was the most common extra-renal manifestation, presenting in 22.2% of patients. End-stage kidney disease (ESKD) was present in 8.7% at baseline. ESKD-free survival probability at five years was 0.80 for paediatric patients and 0.57 for adults. ESKD-free survival was negatively influenced by CFH, C3, or CFI variants. This study highlights the historically poor prognosis for untreated patients with aHUS. The UK population of the Global aHUS Registry represents a valuable research cohort with comprehensive demographic data and high genetic characterization. These findings underscore the importance of early aHUS identification and intervention to prevent ESKD and improve patient outcomes.