Getting into US universities

I only listed unis that meet full financial need.

yes...and you still need good grades to get in!

On the SAT and ACT exams or on my GCSE's ?

Both.

But what would be considered more important the SAT's or GCSE's ?

From my experiences a high SAT will compensate somewhat from lower qualifications, particularly if you take an SAT subject test in a subject you got a B or below in at A Level.

However, arguably your qualifications and not standardised testing is more important as they are a better reflection of your consistent ability instead of just a one off.

It's 'holistic' but you still have to be almost perfect to be considered particularly for those choices.

I was wait listed from my first choice because of my somewhat low ACT score. I had all As and A*s at GCSE and As at AS. I've retaken and its now very competitive, but it's just a tad too late.

Bear in mind unlike in the UK, US universities don't care how many times you retake the SAT/ACT so its your to your advantage to retake (funds permitting).

Can I ask where you applied, or is that too personal?

Barnard (1st choice)
and then a mixture of Ivy Leaguers and Liberal Arts colleges (mostly womens colleges come to think about it)

So how did you apply? Via a ucas like site, or directly?

It's all done via the Common Application:

https://www.commonapp.org/

Is that just for public unis or private ones too?
How much does it cost?

Its for 99% of all universities in the US and a few UK ones too apparanty. It depends I forgot how much I had to pay to send it to Barnard but I applied for a fee waiver for all of the others. Yes bare in mind its expensive even applying to US Universities. You have to pay to send test scores as well if you don't do it at the time you sit the tests. Also if you apply for financial aid you have to pay to send forms.

So do you suggest I only apply to a few for economic reasons.

No. I would suggest you apply to about 10, it increases your chances dramatically. Consider (if you really serious) Early Decision as well (the percentage of admitted students is a lot higher than under regular). You can apply for fee waivers for the Common Application but its about how much you want this.

I have paid a lot to apply to these universities despite having a lot more being paid for me and its likely only a few will accept me.