With Brexit, the United Kingdom (UK) left the European Union’s (EU) delegated trade model and is able to independently set up its own free trade agreements (FTAs). This is considered to be one of the major advantages of exiting the EU and its Customs Union. FTAs are a priority of the UK government’s economic approach post-Brexit linked to prosperity and well-being. Yet trade has distributional consequences. Its benefits are dispersed while its costs are concentrated. Against this background, this paper examines how individuals form preferences on FTAs. Starting from the premise that signing FTAs is a new and hard political issue, i.e. high complex with undefined outcomes, we posit that to form their preferences individuals employ heuristics related to their subjective perception of whether the country will have the upper hand in the negotiations. The effect of heuristics is amplified through Brexit-related affective polarization. We test our hypotheses, using original observational and experimental data from a survey representative of the UK population, carried out by YouGov in July 2019, which confirm our expectations. Our findings have implications for the study of the role of heuristics in opinion formation on highly complex issues.