I attended in August 2018. They put my photo in the 2019 brochure actually lol.
I didn't do engineering, I did physical sciences, so I can't say what that'll be like exactly, but we had a few days of lectures where we actually learned things that were new, (like i'm only learning them in school now in the second term of year 13), and then a few days on a presentation in the first week. The second week we did a team project that wasn't academically related to our subject as we worked in teams across the three streams but it was really interesting because we learned about like, design and worldbuilding and things. But the second week was not academically intense or challenging.
There's a lot of free time and time dedicated to doing very touristy things. But you basically have a nine to five day, plus lunch in the middle, and then they fill the rest of the time with things around London or around the university. The weekend in the middle is a weekend. Saturday was Brighton, and Sunday was Leicester Square and the talent show if I remember correctly.
The Global Summer School, for all the intensity of the application, what with the personal statement and academic record, is not heavily academically rigorous. But it is academic, just not in the way you might expect for the second week. The first week is like a taste of university teaching basically.
That being said, it was probably one of the best two weeks of my life. Highly, highly recommend it to everyone. Maybe not you though OP it doesn't sound like what you're looking for lol!
If anyone reads this who wants to do life sciences - it's barely anything but medicine. My roommate was in the life sciences stream and she wants to do biology or something to do with plants and it was pretty boring for her, so keep that in mind.
I uploaded my timetable for you to look at.
Week 1:
https://imgur.com/sNUvd0cWeek 2:
https://imgur.com/jFXa8zC