Hi ML07,
Your A-level grades are great and they will not negatively affect your applications for most firms (even those with minimum grade requirements). Over recent years firms have started to remove A-level grade requirements entirely, showing that firms tend to place more value on other elements of an application, such as university grades and extracurriculars.
Saying that, there are still things you can be doing in the meantime to improve your future applications. Aside from university grades, the most common downfall that candidates show is a lack of commercial awareness and real-world commercial experience. You can get creative when demonstrating these things, and the more 'outside-the-box' examples will have the advantage of catching the reader's (ie. graduate recruitment's) eye. For example, you might join a university society and gain a position on the committee, volunteer for a student-led charity in a leadership position, or set up your own business. The most important thing to remember here is how you frame your experience, and this often makes the difference between a 'good' and a 'bad' application.
I would advise against retaking your A-levels during your gap year. In your applications to law firms, you will have to disclose whether you achieved your A-level grades on first attempt, or on retaking the exams. As your current A-level grades will not be a barrier in your applications, it would be a waste of time and money to retake the exams.
Let me know if you have any questions, and I hope this helps!