I currently take English Literature, History and Spanish (Y13), you won't be disadvantaged if you don't pick one or the other since you can pretty much do a Law degree with any subjects within reason, personally I think if you want to be stronger at essay writing maybe go for English Lit but a language is definitely a useful subject to have especially if you want to build on communication skills and overall awareness (plus there is also a small essay writing component in Spanish). Based on this, I would say pick whichever subject you enjoy because it doesn't matter how good you are at GCSE but if you are able to maintain a passion after the jump to A Level and achieve the best grade you can. Alternatively, most sixth forms/colleges will let you pick 4 and you can drop one maybe 2 weeks in so you could also try them both out and see later? If you have any other subject-specific questions I'd be glad to help!