STEM supracurricular summer programmes

I'm in Yr11 and I want to increase my chances of going to russell group (mainly oxbridge) unis. I want to study Computer Science or possibly Engineering so does anyone have an recommendation of summer schools/programmes or work experiences they did in yr11? Also, I attend a grammer school and am of a socio-ecomic standard that isn't eligible for support most of the time.

Theres the cambridge sutton trust in y12 and for supercurriculars there is
https://www.undergraduate.study.cam.ac.uk/files/publications/super-curricular_suggestions.pdf
and there are some "suggestions" from a recent applicant at
https://www.thestudentroom.co.uk/showthread.php?t=7424772
Summer schools can be fun to meet other kids but you only learn so much in 5 days (or so). So the key thing is to do some stuff yourself.
Compsci has been very competitive in the last couple of years, something like 1 in 20 for cambridge. Engineering less so. But the first things to do are get a decent set of gcses (so dont really worry about this until afterwards), skill up your maths for tmua, esat, .... and make sure you hit your a level grades. Get some computing experience so you can talk about it on your personal statement, theres euclid project, adventofcode (I like for a variety of reasons), .... and talk about it in an interview are the most important things.
Work experience doesnt necessarily help that much with supercurriculars, but do it if you want, just do the supercurriculars as well.

Thanks!