Achieving the net-zero target of the United Kingdom requires substantial greenhouse gas removal (GGR) in addition to emission reductions. Biochar, a stable carbon-rich material produced through biomass pyrolysis, is an established GGR method. Straw is abundantly available in the UK and presents a viable option for large-scale biochar production. However, uncertainties regarding its feasibility remain, particularly concerning costs, spatial constraints, and facility construction. Here, we use a spatial model integrated with life cycle assessment and technoeconomic analysis to estimate the marginal cost curve for net carbon sequestration through straw-based biochar production in Great Britain (GB). Our findings reveal that straw-based biochar production in GB can achieve 0.6–1.9% of the UK’s 2050 carbon removal target at marginal costs below £75 per tCO₂e. If higher-cost options are also included (i.e., without the £75 per tCO₂e constraint), the total potential removal increases to 0.8–2.1%. The marginal costs are significantly influenced by the price and availability of feedstock, the value of byproducts, and the biochar yield. Our integrated spatial model helps identify optimal feedstock supply and production strategies, reducing costs and uncertainties in net carbon sequestration for biochar systems. This study elucidates the challenges and limitations of utilizing straw for large-scale biochar production in GB to support the climate change mitigation pathway.