With a legally binding commitment of 80% CO2 reduction by 2050, UK’s energy system warrants a deep decarbonisation drive within decades. Past UK energy system transition scenarios approached this transition with a supply-centric approach. In electricity that often meant centralized low-carbon generation options, such as nuclear power plants and offshore wind farms, meeting majority demand. In this study, using a suite of bespoke end-use demand and energy supply infrastructure expansion planning models, we present two transition scenarios aimed at decarbonisation. The first scenario envisions high electrification of heating and transport services and powering the energy system with low carbon electricity, while the other envisions a balanced uptake of energy conservation and efficiency, electrification, microgeneration and large-scale renewable energy. The analysis focused on electricity and gas demand and supply in the island of Great Britain within the UK. When compared with a reference scenario, the balanced approach is found to have the higher potential to reduce overall energy demand along with the demand for electricity and gas, whereas the electrification approach increases total and peak electricity demand by 35% and ~90% respectively over reference by 2050. Supply side implications to meet the demands are tested with nuclear, CCS, offshore wind as well as a balanced mix of major supply-side options. The chosen mixes in the two scenarios could largely meet the power sector decarbonisation goal with varying cost, stranded asset, governance and supply chain implications.