Legislation to decarbonise energy systems within overall greenhouse gas reduction targets represents an immense and unprecedented energy policy challenge. However there is a dichotomy between this level of policy ambition and prior modelling studies that find such targets economically, technologically and socially feasible under idealised ―first-best policies. This paper makes a significant contribution to current analytical efforts to account for realistic ―second-best climate mitigation policy implementation. This is achieved via a technical classification of secondbest common mode issues at a detailed national level: both internal (behavioural change, infrastructure implementation) and external (new technologies, resource availability). Under a combina- tory second-best scenario, meeting targets greater than a 70% reduction in CO2 by 2050 entail costs above a subjective barrier of 1% of GDP, while extreme mitigation scenarios (>90% CO2 reduction) are infeasible. These high costs are equally due to disappointing progress in behavioural and technological mitigation efforts. Expensive second-best mitigation scenarios can still rely on extreme assumptions including the full deployment of the UK‘s offshore wind resource or the complete diffusion of energy efficiency measures in end-use sectors. By demonstrating the fragilities of a low carbon energy system pathway, policy makers can explore protective and proactive strategies to ensure targets can actually be met. Additionally, systematic analysis of failure in stringent long term decarbonisation scenarios teaches energy analysts about the trade-offs in model efficacy vs. confidence.