I promise you, you most likely have absolutely nothing to worry about, these things happen! Most background checks will not go further than University degree level, unless e.g. your company was sponsoring their grads to complete a qualification such as ACA , which has A Level requirements. But since your company did not set any specific A Level requirements, and you're not in finance/law, you'll be fine.
If you are asked to take part in a background check which asks about your A Levels, put down your results correctly, and for your own peace of mind - you could also email your company's HR and briefly explain that you've noticed the small error in your CV. Even if you aren't asked to take part in a check, you could still email and explain if you'd like. It shouldn't affect your offer, since they didn't state any A Level requirements.
This might be a good idea to do if there is a huge discrepancy between your actual A level result and the one on your CV, e.g. if you got a D but accidentally stated that you got an A*.