In the UK and across the world there are thousands of summer schools offering courses in English, often with a focus on an activity such as computing or a particular sport. The quality of these courses depends to a large extent on the quality of their teachers. This varies not only from course to course, but can also vary from year to year in the same school. For this reason, it is not desirable for me to recommend any particular summer course.
Instead, I recommend visiting the British Council website, and using the Search functionality in the page linked to below in order to find a suitable course for your age, interest and language level. All courses listed on the British Council site have been accredited by one of the various language learning accreditation organizations, so you can be sure that the school will meet certain minimal standards.British Council: Accredited English language centres
Good luck with choosing a summer course. And remember: The most important thing is to have fun and learn something new - not just English.
[ Visit my other page with more suggestions for vacation learning. ]