I would suggest A-Levels. I applied to LSE for BSc (Government & Economics ) in 2008 , with my Access to Business studies. They asked me to take their entry test, which they would have not if I had A-Levels. Anyways got rejected by the end of March. Upon query they told me that my marks were not the highest. Try Ealing College or Westminster Kingsway College. Both of them offer A-Levels for adults.