US Citizen living in the UK applying to college in America?

I am an American Citizen as I was born in California, lived there for three years then moved to London. I am also a British citizen. Obviously, I will only have British qualifications (GCSE and A-Level); I will also be taking the SAT but I have no GPA and also haven’t got the same opportunities with extra-curriculars that an American Student does. Apparently, I am not an international student but an out-of-state student. So will I be assessed with other students from the UK/Europe or US students? This really makes the application process more confusing, especially when it comes to financial aid and scholarships. Can anyone provide any resources for me to go to as my situation is quite unique and I can’t seem to find anything. Any help would be appreciated.

Good US colleges can figure out your academic standing from your A levels and SAT scores. Teacher recommendations will be crucial. Coach your teachers way ahead of time as to what’s required; include your resume/vita with a sample letter. You will need to fill out the FAFSA (all colleges) and the CSS Profile (Oxbridge-type) for financial aid. You will likely be assessed with other US applicants: good thing, since colleges have a limit on international matriculants to about 10% of the class. You can contact your preferred schools directly with financial aid and application questions. If you are applying to public universities, you are considered instate if your parents vote there, work there, own property/pay taxes. Most state universities will consider military or diplomatic families to be in-state. All public colleges can give you a determination of in/out state before you apply. You may wish to take US college classes online before you finish high school; those grades and teacher recommendations will help round out your application.
You asked for resources: there’s no one size fits all guide, so you will have to contact each school directly with questions (just email them.) For national scholarships, some are open to US citizens who are nonresident, some are open to only US citizens who attend a public school in the US (so not homeschooled or private schooled.) It varies enormously.
Do you have a school list? Have you run some financial calculations on school websites? Do any other family members attend college currently? Get your parents to start talking finances with you right now, and start pulling together documents.

You will be assessed against American students, not against international students. However, because you don’t pay taxes in any state, you will not automatically get that state colleges’ in-state tuition rates. You will be eligible for merit aid, so do as well as you can on the SATs. Some states, such as Alabama and Montana, may give you in-state tuition based on your test scores. California and Illinois won’t.

American students just don’t have magic opportunities for ECs, that is not something to repeat. Do you have a job? A hobby? a sport?
Your situation is far from unique. Lots of expat kids are in your situation. The most important point here is $$. As you have UK rates and a 3 yr degree option, that would be hard to beat, if you are a student that gets merit money in the US, you are a student that can access terrific UK schools.
Have your parents run FAFSA and NPC numbers. Make sure you add in health insurance and travel costs. Are your parents still doing US taxes?
What are your O level results?

(It’s GCSEs now in the UK, not O levels - theyve only kept the O level name for the international version.)
What stage are you at now - entering upper 6th? If so, what A levels are you doing and what predictions do you have? They will look at your GCSE results.
They will assess your application academically based on other UK applications, but it will be easier to get in than it would for other internationals. What can your parents afford to pay, and does that match your expected contribution?

Nope- genuinely thousands of US students are in secondary schools around the world and applying to US universities! That’s actually good news: US universities know how to deal with this. Better again, you are in the UK, one of the most-familiar international educational systems. They all know about GCSE’s & A-levels (& predictions), and they know what sorts of ECs are usual in the UK.

Figure out what schools a good fit for you- in terms of finances, of stats, of interests. Start with finances: what can your family spend? In-state/OOS only matters for public (state) universities. Money for college comes in 2 main flavors:

Financial aid, where the college gives you some amount of money. “Meets full need” places will meet the difference between the cost of attendance and what the school thinks you need based on your family’s finances. A few places (mostly highly selective) are “need blind”; most are “need aware” (that is they take your financial aid needs into account in the admission decision). For public schools you will be treated as an OOS student, for private schools as a domestic student.
and
Merit money, where a college encourages you to come by offering scholarships (though there are a lot of different names). The more they want you (and particularly your stats) the more money. Thus, you get the most money from places where you are at the high end of admitted students. There are some super-selective named scholarships out there as well.

Then stats: what level of competitiveness are your SAT / ACT and predicted A levels?

Finally, what do you want from your college experience? Fiske Guide is often mentioned here, but there are lots of resources.

The other financial aspect to pay attention to is the dwindling pound. It sounds trivial but it just won’t be.

Also remember once you are of age to do US tax returns, that is a US citizen requirement that kids may not be aware of.