You could always be applying to sixth form in 2019.
What was the reason for your poor attendance?
College/school is easier and its free.
I have looked at A level politics fro AQA and that would be suitable for home study as it looks to be 100% exams.
https://www.aqa.org.uk/subjects/government-and-politics/as-and-a-level/politics-7152/specification-at-a-glanceAre you expecting to do this for Summer 2019?
AS or a whole A level?
Pick exam board
1. Download specification.
2. Decide if you want to pay for an online course or get text books and work out the price [ £300-500 or £50-100 for books]
3. Decide if you need essay feedback.
4. Arrange for exam fees and venue. Could be £300-450 to sit.
Self study imo is more difficult and you have to be well organised and motivated.
All depends what you are like as a self studier.