Hi. I don't do A-Level, I do the Scottish equivalent (Highers), but I want to put my two pence in. I take all essay subjects, and because I enjoy them and essay-writing is my thing, I've not found it any more difficult than if I were taking a different combination.
I take Higher Classical Studies, and the Higher and A-Level courses seem quite similar.
In both, you study literature in translation. As someone who also loves English, I've found it so interesting to explore the plays in their historical and cultural context and consider how this influenced the key themes and the portrayal of these themes. Since you're planning on taking English Literature, you might really enjoy this element of it.
In A-Level Classical Civilisation, there is beliefs and ideas content, and for this you could study things like religion, love, Athenian democracy, and politics in the late Roman Republic. In Higher Classical Studies, I've been studying the last two from that list, and whilst it's a lot to wrap your head around at first, I just love it. It shows you the ways in which a city state and an empire ran and the intricate politics within it. If your school does religion, you may enjoy this since you're planning to take Religion, Ethics, and Philosophy - however I feel there's ethics to be considered when studying the politics too.
The skills that it develops are useful for law (although you will gain these on English Literature and Religion, Ethics, and Philosophy too). I would say it compliments and goes well with the two subjects you know you want to take to A-Level.
I also feel that if you're not too confident at maths (you are doing really well though), then I don't recommend taking it to A-Level. That would cause you extra pressure on top of the already high workload, which I don't think is sensible. From experience (both self-earned and witnessed), taking a subject that you're not confident with further can make you more likely to struggle with it, even if you're more than capable of doing well.
Sorry for such a long answer. I hope this helps, and if you have any more questions, feel free to ask me. 😊