For full context: I was in the process of applying to the Ivy league + others a year ago, decided not to apply in the end but others in my school did managed to make it (although not many). This was a private school in central London.
I find your desire to apply to the US commendable, I'm sure that you will likely face many obstacles on this road so I wish you a lot of luck regardless.
Here is the standard information:
I would seek out as much information as possible.
Using reddit (r/applyingtocollege & r/collegeresults) will give you an idea of the profile people have and where they end up going.
Here is a Notion website written by someone from the UK who then went to an Ivy League:
https://getintousunis.notion.site/How-to-get-into-the-Ivy-League-other-Elite-American-Universities-as-an-International-Student-ca44149dfa9240ab9ce32d9cae0b2e67The rest below here is simply my opinion:
Most applications have a general major or identity the applicant is going for (ie CS, Physicist, Medic). As a general rule, I would strongly, strongly advise you not to apply as a "CS person" at MIT or Stanford.
The competition is simply too great, even for the brightest ( from personal experience but also see r/collegeresults for lots of examples). This does not mean that you cannot study CS there, but you can make your application profile under something else (ie Physicist is specifically into Quantum Cryptography - technically can be CS but you are now passed off as a physics major). Anything Environmental or more humanities leaning is even better - I know this sounds like cheese but this is the state of affairs we are in when it comes to MIT/Stanford/CalTech.
ECs are always about impact, so to each of the things you mentioned it is important to ask yourself how much impact you've done in tangible numbers, this makes it easier to stand out (e.g A non-profit is not that uncommon for applications, it is a meme in the applicant community at this point).
With this current profile, I would highly recommend that you consider colleges like Yale, Duke, Brown, Cornell rather than just MIT, you have a better chance as these are less associated with STEM and are more likely to appreciate your application.
I would also suggest that you consider applying to some lesser known schools to hedge (unless UK is your backup).
Amherst college is my personal favourite, it is a top liberal arts college that is aid blind in admissions for internationals (IMPORTANT), it kind of like Oxbridge in it's academic feel & more of a close community than large Unis (MIT is huge).
Please do not interpret this as discouragement from applying to MIT (I had to deal with my fair share too).
I just believe that you should identify what you want out of this application process and put forth a full strategy to achieve it. (companies are making millions just creating strategies for kids to get in).
I'm happy to respond if you have any further questions, I've tried to be concise but this application process is quite overwhelming at first glance.
Good luck in all of your applications.