Hi. I'm sorry to tell you this but if you have straight 5s, you are at quite a disadvantage. They state on their website this:
"Have excellent GCSEs or S4 qualifications. This means you have achieved at or close to the grades below. In England and Northern Ireland: at least eight GCSEs at grade A or 7 or above, or near this level. In Wales: at least eight GCSEs at grade A or above, or near this level. In Scotland: at least six B passes at National 5 or above, or near this level."
You would need a lot of widening participation criteria and/or extenuating circumstances for your application to be competitive. It also depends on how competitive the applicant pool is, which no one knows at the moment. However, based on previous years, it's very competitive. I'd recommend that you also apply to other UK summer schools and programs as well, just to maximise your chances of getting onto a program - I don't know of any other US programs so you'd need to look into that.
If there are any tasks or interviews that you have to do, do the best you can to maximise your chances.
I didn't apply for the US program, but for the UK Summer School program, I got in with 998887766 and extenuating circumstances. It might be worth posting on r/6thForm on Reddit to see if anyone else has either in the same situation as you or has been before.
I do hope your application goes well though and that you are able to get in. Good luck