AbstractUncertainty is inherent in modelled projections of bioenergy with carbon capture and storage (BECCS), yet sometimes treated peripherally. One source of uncertainty comes from different climate and soil inputs. We investigated variations in 70‐year UK projections of Miscanthus × giganteus (M × g), BECCS and environmental impacts with input data. We used cohort datasets of UKCP18 RCP8.5 climate projections and Harmonized World Soil Database (HWSD) soil sequences, as inputs to the MiscanFor bioenergy model. Low annual yield occurred 1 in 10 years as a UK‐average but yield uncertainty varied regionally, especially south and east England. BECCS projections were similar among cohorts, with variation resulting from climate cohorts of the same database ensemble (3.99 ± 0.14 t C ha−1 year−1) larger than uncertainty resulting from soil sequences in each grid block (3.96 ± 0.03 t C ha−1 year−1). This is supported by annual time series, displaying variable annual climate and a close yield–BECCS–climate relationship but partial correspondence of yield and BECCS with maximal soil variability. Each HWSD soil grid square contains up to 10 ranked soil types. Predominant soil commonly has over 50% coverage, indicating why BECCS from combined soil sequences were not significantly different from BECCS using the dominant soil type. Mean BECCS from the full climate ensemble combined with the full soil sequences, over the current area of cropping limits in England and Wales, is 3.98 ± 0.14 t C ha−1 year−1. The bioenergy crop has a mean seasonal soil water deficit of 65.79 ± 4.27 mm and associated soil carbon gain of 0.22 ± 0.03 t C ha−1 year−1, with bioenergy feedstock calculated at 131 GJ t−1 y−1. The uncertainty is specific to the input datasets and model used. The message of this study is to ensure that uncertainty is accounted for when interpreting modelled projections of land use impacts.