I recently looked into this topic as I was keen for my son to attend a summer school between the first and second year of college. I know it’s more of an American thing, but I actually found some great options in the UK too. My son was really keen on studying medicine at university so we looked specifically for medical summer courses that would look good on his application.
After some searching, I found a company called Immerse Education. They offer academic summer courses for all sorts of different subjects. My son chose the medical course, but they had some other really interesting options and even run some of them from Cambridge. If you’re looking for a course that has a heavy focus on academics and not just ‘summer fun’ then I would definitely recommend one of their courses. My son had a great time, but he also learnt a lot.