LONDON — In the 2016 Brexit referendum and the 2017 general election, Henry Pritchard was not eligible to vote. He was too young.

Now, having surpassed the legal voting age, 18, and eager for the government to change its course on leaving the European Union, he is desperate for an opportunity to have his say.

With Prime Minister Theresa May departing and the Oct. 31 Brexit deadline looming, choosing the country’s next leader is one of the most important decisions in a generation or more to confront Britons. But in the coming week’s election, Mr. Pritchard once again finds himself excluded.

[Update: Boris Johnson is set to become U.K. prime minister.]

“This is my future, Brexit is my future, but I am not allowed to participate in the process of deciding what that future might look like,” the 19-year-old gap-year student said in a phone interview.