Any exam centre (all secondary schools or colleges in England, Wales & NI plus some private places that specialise in it) can accept private candidates. You pay them and they organise everything. As a previous poster has said, you'll almost certainly need to get the practical endorsement for Biology and Chemistry and that limits the places that will accept you and also makes it much more expensive - £1,000 to take Biology or Chemistry including the practical endorsement is not unusual and it can be more..