it is quite a big leap from gcse to a-level, but if you're prepared to work hard and practise a LOT it will be helpful, especially if you want to go into econ at uni, as most of the top unis will generally require maths a-level too
my friend got a grade 7 at gcse and is currently doing better than me in a-level maths (i'm in y13) because he works insanely hard and does hundreds of exam questions.
i do have to ask though, why are you doing core maths?