The UK is committed to achieving net-zero emissions by 2050. The built environment is a major contributor, responsible for around 25% of national greenhouse gas emissions. Accurate quantification of emissions is, therefore, essential for guiding sustainable design and meeting national climate targets. Emission factors per monetary unit (e.g., CO₂e/£) are particularly useful, enabling businesses and policymakers to estimate emissions across various contexts. These factors are applied, to name a few, in quantifying Scope 3 emissions (including indirect emissions within the product supply chain), assessing product emissions when physical data are scarce, informing emissions reduction strategies, and supporting climate policy development at both sectoral and national levels. However, current emission factors are derived from diverse sources that use different methodologies, assumptions and data inputs, resulting in discrepancies that undermine reliability, and thus hinder sustainability assessments. Hence, this paper examines and highlights these discrepancies by reviewing available MRIO datasets for the UK, focusing on construction materials. It explores dataset methodologies, sector definitions, system boundaries, and underlying assumptions. The paper, also, compares emission factors across different MRIOs. These are extracted directly, but when unavailable, emission factors are calculated using the pymrio Python package. To ensure cross-dataset comparability, harmonisation was applied, such as the conversion of emission factor values to GBP (£). The comparison highlights significant variations in emission factors across MRIO datasets, largely due to differences in methodology and assumptions made. These findings raise important concerns about the reliability and robustness of emissions quantification. The study concludes that stronger alignment between these datasets, more rigorous validation and systematic cross-comparisons are essential to ensuring sensible sustainability assessments in the UK construction sector and supporting climate action aligned with national targets.