Yeah but utilitarianism requires to account for the total sum of the utility, not just the utility to an individual or their job. That is definitely not what old Bentham had in mind.
There is much to gain from learning things, whatever one’s job. Case in point, being able to have better social awareness, understand political discourse, and detect snake oil and populist lies in order to avoid voting for parties and policies that ultimately damage one’s interest and the interest of the collectivity. Which is exactly what millions of people did in 2016. The book What’s the matter with Kansas, by Thomas Frank, comes to mind. Being made a fool by dishonest politicians is the opposite of utilitarian.
It appears that you did not find value in your education, and that is unfortunate, but it is not the norm, and perhaps you may still find it later on. While I agree that the state of education in the UK and many other places sometimes legitimates doubting in its value, statistics clearly point to it providing a definite advantage in many areas of life including health, wellbeing, life expectancy, social status and more. And that, again, is utilitarian :)
Of course, being educated doesn’t necessarily mean being formally educated. As you pointed out, the plumber _was_ a scholar in his own way, after all :)