What are my chances of getting onto a training contract?Watch
No one here (or anywhere, for that matter) has a crystal ball through which they can divine the probability of you obtaining a training contract. Whether you eventually get a training contract depends on a hundred and one different factors, including market conditions at the time, the attractiveness of your application relative to those of other applicants, whether lady luck likes you, whether the people reviewing your application ate a good breakfast that day, etc. In short, your chances are somewhere between 0 and 100. The only two certain things I can say are: (1) your A levels are fine; and (2) getting a First does not guarantee a training contract.
You are better off doing your best to improve your application - e.g. acquiring relevant experience and talking to law firms and recruiters rather than strangers on a forum - than fretting over probabilities like this.