Oooh what a tough question! I think you have a number of options here:
1. Try seeing if a nearby school does Politics A Level and asking if you could join their classes for just this
2. See if there are any course online that allow online learning of alevels
3. Try doing things surrounding this in your spare time such as volunteering at your local constituency office and getting involved with grassroot organisations
As someone who did Politics A Level and then went on to study Politics at Uni, I can honestly say it didn't give me as big insight as working alongside my MP and doing Model UN. Obviously I understand why you want to do it but thing such as History and Sociology can give you the transferable skills you need.