Global food systems are increasingly characterised by structural changes as environmental pressures and shifting consumption patterns reshape the foundations of food production and supply. Within this context, protein production plays a central role in determining agricultural emissions, land demand, and resource use. Countries that depend heavily on food imports while maintaining livestock-dominated agricultural sectors face particular challenges in balancing domestic food security with global environmental responsibility. The United Kingdom represents such a case, especially within the rapidly evolving post-Brexit context of heightened political and economic uncertainty.

The aim of this research is to examine how structural transitions in the UK protein system influence sustainability outcomes and food supply across interconnected domestic and international contexts. It develops a set of interconnected, protein-specific multi-regional input–output modelling frameworks to assess how changes in trade, demand, and production reshape economic and environmental outcomes. The research evaluates system-level responses under interacting structural adjustments.

The findings demonstrate that sustainability outcomes in the protein system are shaped by interacting structural drivers rather than single interventions. Reductions in livestock consumption and production generate substantial domestic environmental benefits such as greenhouse gas (GHG) emission reductions and land use savings. However, domestic improvements do not automatically translate into global gains, as sourcing adjustments and trade configurations may redistribute environmental pressures to other regions. Furthermore, land-use reallocation following livestock decline emerges as a critical mechanism through which environmental objectives and food-security considerations are jointly determined. The sustainability consequences of protein transition depend not only on demand shifts but also on how released land is governed and strategically allocated. Methodologically, the thesis advances the application of multi-regional input–output analysis to protein-system research by refining sectoral representation and integrating demand- and supply-side structural adjustments within a unified analytical framework.

Overall, the research provides a comprehensive framework for understanding protein-system transformation in an increasingly interconnected and uncertain global environment. While focused on the UK, the analytical approach and insights offer relevance for other import-dependent countries seeking to align domestic food security with global environmental sustainability. The findings highlight the importance of coordinated governance, environmentally responsible trade policy, and strategic land-use management in shaping coherent and globally accountable protein transitions.