Hello from Wales! I was to do the extra maths GCSE but I decided I could never do or think of maths as any further than a necessary subject I needed to get through life. Past GCSE, for me I didn't need it.
However, universities, I believe, can see GCSE results; they need to see whether you passed English and maths GCSEs. Maybe places like Oxford will take into account sometimes your GCSEs in competitive subjects like PPE but it's unlikely. I got an A in numeracy last November, so you're already better than me! Since extra maths is on the pass, pass* and merit system, I I think pass* or merit should suffice if ever they want to take a look at your GCSEs. But I had a B in history last year so I wouldn't sweat it. (I am in Set 1A, I just hate maths)
A quick question, which other A Levels are you taking? If you want to take Maths and Further maths and get an A and B respectively, Oxbridge will never care about maths GCSE results- a B in further maths is extremely impressive.
I might be going to Oxford for English to study Jane Austen lol, so if I decide Modern languages or Welsh degrees are not for me, see you there!