This UK-based retrospective analysis describes real-world treatment patterns and outcomes in 175 patients with accelerated (AP, n = 69) or blast-phase (BP, n = 106) 'Philadelphia-negative' myeloproliferative neoplasms (MPN-AP/BP) diagnosed between 2013 and 2025. Median age at transformation was 71 years. With a median follow-up of 45.2 months, median overall survival (OS) was 14.9 months, significantly worse for MPN-BP (6.7 months) versus MPN-AP (25.3 months). Treatment selection was heterogeneous across centres. Intensive chemotherapy (IC) improved outcomes only when followed by allogeneic haematopoietic stem cell transplant (allo-HSCT) (median OS 24.7 months). Ruxolitinib-based regimens, particularly combined with azacitidine, showed acceptable activity in AP (median OS 27.2 months). Venetoclax-based regimens achieved a median OS of 14.9 months across the cohort. Multivariable analysis identified IC and venetoclax-based therapy as independently associated with better outcomes, reflecting patient selection, while TP53 mutations predicted inferior survival. IC carried high rates of febrile neutropenia and sepsis; venetoclax was associated with prolonged cytopenias. This study confirms the poor prognosis of MPN-AP/BP, the absence of a unified UK consensus approach and the need for improved therapies and prospective studies to determine optimal treatment approaches for this challenging cohort.