Hi, not yet but due to at some point in DD's Y12, subject to UK test centres re-opening. What do you need to know?
If at the start of your DC's prep and your school does not provide US application support, take a look at Fulbright Commission for basic detail on how to apply, funding, possible financial support etc.
Get your DC to register with SAT/College Board. Here he/she will be able to access lots of free test preparation materials, practise with Khan Academy linked stuff in readiness of how tests work/what subject matter will be included etc.
Decide which subject papers (it any), on top of the SATs your DC may wish to take.
Look at your prospective colleges' websites. Lots will have changed now since CV-19 impact, both for domestic and international students. Some prestigious institutions have/are making SATs/ACT testing requirements optional, but may still be viable for international students as we have different high school exams and do not capture on-going GPA data like they do. Again, all may change after this year...
Ensure your DC establishes strong relationships with their nominated subject teachers providing references!
Think about the essay writing skills - a big part of the application process.
Sorry this is a bit of a download dump of stuff; you may have already got this all covered! Good luck.