If the idea of preparing for an AP test wthout the support of a classroom teacher seems daunting, the UK system as a whole is not a good fit. And thats okay! Horses for courses.
Similarly, if researching the websites of classics departments for every single school of interest and taking them at their word, trying to immerse yourself into a totally different mindset as far as admissions are concerned, is not how you want to approach your college search, it is not a good idea to start college abroad.
If you want to have a chance at applying for a classics course in the UK, I’d recommend to take the following AP tests: Latin, English lang, Euro History, Italian, whether you find a class for it or not. Retake the ACT or SAT. Make sure to pick a range of universities for your UCAS list, eg apply to a few departments that require only a 29 ACT and Bs/4s for the subject tests taken. Good luck!