That squares with what I have heard before about UK university degrees being shorter than US degrees. What do you do with the rest of the year?

Internships. Or work, or study. Up to you. You have an informal exam when you get back and if you annoy the profs enough they'll kick you out for failing it. But that's pretty rare.

Time spent per year is roughly equivalent. UK degrees are shorter than US degrees because we spend only three years at university for a bachelors degree. Depth is similar but there's less breadth - we only study our major, so as a CS student all my classes were taught by the CS department apart from a handful of math classes.