I don't believe that the topic itself is of any real significance for your prospects of admission. Though this is only a guess, I believe that they look at your writing style and reasoning, not the topic, as they are probably aware that not everyone has written on what they want to pursue their BCL in. Plus, you don't have to decide which subject you're gonna take, so how are they going to know whether the topic is relevant to your studies (well, most likely you do mention that in your personal statement, but still ...)?

Anyway, I wouldn't be worried about any of that, I want to study intellectual property law and EU competition law, and have submitted a sample on juries in English Law. Completely unrelated topic-wise, but law is law and they are looking for your legal reasoning and not what you've already achieved in the field. After all, for that they can look up your CV, your hitherto studies etc.