Your chances will be very good with KCL if you can get a 5 in your AP Econs, since you already have very good grades in your two SAT Subject and SAT Reasoning tests. You can combine the SAT-S with the AP in your application.
Not too sure it will be same for UCL because it is an elite university for Economics. It is one of the competitive universities to get admitted in for an Economics course.
KCL Economics course, on the other hand, is a relatively new course offering that I don’t even think has produced its first graduating class yet. It previously only had a course called Political Economy, which was highly competitive to get into, but was more slanted towards Politics in its teaching.
You will have highly competitive applicants to both universities for the course just because of the prestige of both universities but I think you have a better chance with KCL.