You realise you'll be covering exactly the same material in a typical foundation year as you would in A-levels (presuming you would take A-levels in Physics, Maths, and either Further Maths or Chemistry, usually)?
A foundation year isn't an extra year of engineering or anything. You will literally be covering the exact same material A-level students cover in those subjects, because that is the content you need to know to undertake that degree course, and aside from possibly a couple of "taster" lectures or sessions (which will be much the same as the content you would see on an open day) it's not going to be significantly different.
I don't really see there is any reason or benefit in taking a foundation year after GCSEs as such. I also highly doubt you would be able to apply for one anyway, with only GCSEs, at 16.