The OCR specification (the main board for Latin and sole one for Greek) is pretty much the same for both languages and the amount of grammar needed is similar. Although if you're self-studying, Latin is probably easier (way more Latin GCSE books, Youtube videos, easier access to set texts etc.), there really isn't much for Classical Greek (I'm doing it now and though I love it, there's very little available online).
Although if you think you could do it, Greek is great fun and unique. John Taylor's Greek to GCSE books are great for all the grammar (he's also written ones for Latin to GCSE) and as cheetobish said, there's the JACT course.