Business Edexcel

I have my a level business exam in a week. I know nothing. I didn’t do it for GCSE, and from having a mix of bad teachers and my general gaps in my knowledge- i genuinely don’t feel as though i could answer a single question at the moment. What’s the best way to revise and what are the most important topics to revise?

Kind of left it last minute didn't you?

As you're really short on time, I would look through the specification and see which areas you are completely fuzzy on:
https://qualifications.pearson.com/content/dam/pdf/A%20Level/Business%20Studies/2015/specification-and-sample-assessment-materials/9781446914694_GCE2015_A_BUS_WEB.PDF
If you feel like you like you don't know the material, look through the textbook and read the relevant sections. You don't need long winded notes, but you need enough to get the concepts.
I would also get revision guides specific for the exam board you are studying under. e.g.
https://www.whsmith.co.uk/products/revise-edexcel-asa-level-business-revision-guide-and-workbook-includes-online-edition-revise-edexcel/andrew-redfern/mixed-media/9781292213217.html
https://www.amazon.co.uk/Revise-Edexcel-Business-Revision-Workbook/dp/1292213213
https://www.cgpbooks.co.uk/secondary-books/as-and-a-level/business/buer71-as-and-a-level-business-edexcel
As you're short on time, pop down to the local Waterstone or WH Smith tomorrow, get back home right away and hit the books. Your aim is to focus on areas you are weakest in based on what you have seen in the spec. The areas you are strong in aren't particularly a high priority.

I would then pick up past papers on Business to work from:
https://qualifications.pearson.com/en/qualifications/edexcel-a-levels/business-2015.coursematerials.html#filterQuery=Pearson-UK:Category%2FExam-materials
Do 2-3 papers each day. Review your answers, and mark it against the mark scheme. Highlight your weakest areas, then correct yourself.
Redo the questions that you didn't do well in in light of the review of your answers and see if you can get higher marks. If you get anything less than 80% of the available marks for the question, redo the whole question. Repeat until you have satisfactorily high marks. The idea during the review is to highlight the gaps in your knowledge.
If you find that you really don't know the subject, use the revision guide (not the textbook) to review your knowledge.
If you ran out of exam papers to do by EdExel and you don't have time to buy exam practice books, do exam papers by AQA then OCR.

Do note, a lot of the material tested is about application. Straight memorisation won't necessarily help; it's about how you use the information in the context of the question. This is why you don't need to be hyperfocus on the details and getting everything down to the T (unlike STEM subjects). The only time when you need to be specific is when you have to write down the definitions of keywords or remembering certain accounting formulas/ratios.

The important topics would be anything highlighted in the spec. Anything not in the spec won't be tested. Otherwise you won't know what you will turn up on the paper.

General tips on getting high marks in Business A Level, if you have time left to implement them:
https://www.youtube.com/watch?v=xdWBh3SxFqo
https://www.youtube.com/watch?v=w2yTDxsTw-M
https://www.caiebusiness.com/how-to-achieve-an-a-in-business-a-level-using-the-business-news/
https://www.youtube.com/watch?v=O5Dymo1Dnug
https://www.youtube.com/watch?v=UVSwt_6GNLI
https://www.youtube.com/watch?v=9GF7H3zHDQk
https://www.youtube.com/watch?v=GgN_TULQ2w0
https://www.caiebusiness.com/get-evaluation-marks-a-level-business/
https://www.youtube.com/watch?v=9RK03oSnfg8
https://www.youtube.com/watch?v=5QEZnQoSuAk
https://www.caiebusiness.com/how-score-analysis-marks-in-a-level-business/
https://www.youtube.com/watch?v=PrZ2PE9Oy3k
https://www.youtube.com/watch?v=jymIT4KhTrg
https://www.caiebusiness.com/how-to-write-a-level-business-essays/
https://www.youtube.com/watch?v=du5uodDMnrU
Watch these on the bus at double speed when you're on your way to get the revision guides, or on your way home.