The grade you need at GCSE to atke A level is set by the 6th form - if you do really well at Foundation and get a 5 then that might be enough for some colleges - but many will require a 6 (and some a 7). Even if you get a 5 and they let you do A level you will have to work your socks off at the start of A level as you will be missing a lot of presumed knowledge.