Mature Student Distance Learning GCSEs and A levels

During year 11 I was unwell so I missed most of the year and left with just 7 GCSEs, A* A A A B C C (for the Cs I was only allowed to sit the Foundation papers). Feeling pretty uninterested and demotivated upon leaving school I went on to the College my friends were going to, to do a Business BTEC - having not thought about my career. On applying to University I realised how much I'd limited my options and could only apply for Business/Finance/Marketing courses. I went to Durham to study Finance (extenuating circumstances for my GCSEs and I got D*D*D* in my BTEC) but I hated the course and dropped out during second term, without taking any exams. I'm 20 now and want to better my education so want to retake the Cs and B I got at GCSE in Maths, Religious Education and Science (but I want to take double Science giving me 9 GCSEs). As I'm classed as a mature student I'd like to take the courses at home instead of a College. The thing is I also want to take A levels so that I can get into University to study Archaeology. I obviously want to get on with things a bit now so don't want to spend too much time studying. Basically I'm wondering if it is possible to study GCSEs at the same time as A2 and AS or whether I'll need to spend a year taking the 3 GCSEs and then the A levels (all distance learning)
Apologies for the long post! Very grateful for any help