I would doubt it matters unless you want to go to one of the 'top' universities. If this is the case, I am told that having the English Baccalaureate (maths, english, sciences, a language and a humanity) is useful, and having as many GCSEs as possible is good, but even for 'top' unis you don't need more than 10. However, if you think that continuing it will harm your grades in other subjects I would say definitely drop it + no one will know on your cv that you dropped it.