Many would argue that Brexit was more about English nationalism than British nationalism.

What makes the UK very complex is the coexistence of two layers of national identity – British layered over English, Scottish, Welsh, (Northern) Irish. Which layer a person identifies with is very often determined by their politics, and sometimes also by religious/cultural background. (British vs Irish identity in Northern Ireland tends to correspond with Protestant vs Catholic religious background, albeit there are exceptions to that generalisation.) At the same time, given England is 85% of the population, the boundary between "British nationalism" and "English nationalism" is often quite vague. Its boundaries with Scottish nationalism, Welsh nationalism, Irish nationalism, tend to be more clearly cut.