What would be the best things to do as an international student to improve chances of entrance into elite colleges?

The university tries its best to give all the international students equal chance and they take into consideration the various types of curriculum that students around the world study in. So, no you will not be disadvantaged in your application. The only difference is that international students have higher fees and its harder for them to receive aid or scholarships.

in order to stand out, try following your passion. Do as many extracurriculars as possible in the field which interests you i.e the one which you want to pursue. The experience you gain will also help you in your essays as they contribute a lot to many applications apart from academics.

If you have a particular college in mind try talking to other international students from the UK who are currently enrolled and check out the mission and vision of your colleges. When you write your application, your personality should match their mission, vision and studying environment. All these things will help you stand out and coupled with high academic scores, your chances will be high to get accepted

Firstly, you do not get disadvantaged because most American Universities expect you to complete high-school with the number of years in the UK. So finishing your A-Levels, will not put you at a disadvantage because you have done the max number of high-school years available.

For standing out;

You should really try your hardest to get a good score on the SAT's with a lot of practice and dedication.

You should also focus on your extra-curricular activities a lot, like MUN, or just what the school offers and what else you can do in your community.