Abstract This paper analyses the effect of trade policy uncertainty in a customs union on export‐platform foreign direct investment (FDI). First, I develop a partial equilibrium framework with heterogeneous firms based on the proximity–concentration trade‐off that involves trade policy uncertainty about trade costs inside a customs union. Second, I derive an empirical equation that I test by exploiting the 2016 Brexit referendum as a natural experiment and using data on manufacturing greenfield FDI projects. The results show that trade policy uncertainty impacted negatively the decisions of firms to invest in export‐platform activities in the UK following the Brexit referendum. Moreover, I find that extra‐EU manufacturing firms preferred EU countries as opposed to the UK for new investments after the 2016 referendum.