I’m an American student. My grades are strong (3.9 GPA/4.5 weighted), my SAT fair (750 reading/680 math), I got a 720 on my biology SAT subject test (intended major), but my AP scores aren’t great. I got a 3 on Art History, English Language, Macroeconomics, US Government, and Human Geography. I only got a 4 on World History and Biology. Can I get in despite my AP scores? Should I take more subject tests to make up for them? Thanks in advance, any advice appreciated! 
Do your homework! Your grades are irrelevant. Your standardized testing, in relevant subjects. is what matters.
For KCL Chem & Bio are required for a bio course; for UCL, Biology + Chemistry, Mathematics or Physics. You will be at a genuine
Subject tests can substitute at both places, but first go actually read the requirements for admission on the websites.
And while you are at it, read the year-by-year description of the courses – I wonder if you know what you are applying for.
Are you taking any additional APs this year which you could realistically hope for better scores in? (Pref the STEM subjects@collegemom3717 tells you are required for this course; nb by math they mean Calc BC not AB.).
As it stands your scores are too low for these particular universities. E.g. for Bio Sciences UCL asks Americans to have scored three 5s and two 4s across 5 APs taken in the last three years of HS, to include a score of 5 in bio, which you have unfortunately already missed.
Tell us a bit more about what you want - is it only London that’s attracting you or might a lower ranked uni elsewhere in the UK work for you? Why the UK in the first place - do you have US backup options? Why biology?
My DD got into both of those schools and we think it was only because of her AP scores, her ACT was just 29 but she had 5 scores of 5 out of her 8 AP classes - we think that is why she got in. She chose to go elsewhere however due to the higher cost of the London schools as well as the living cost in London which impacted the financial maintenance requirements to obtain a student VISA.