You are correct that law firms will query why you want to do law. They will also question your motivations for studying English.
I read English and History at the University of Sheffield, so also had to address these issues.
Why law and why commercial law in particular?
This is a question that will vary for everyone. For me, it was not only the intellectually rigorous aspect, but the pervasive aspect of the law in everyday life that was appealing. Commercial law will depend on your interests so think about whether any extra-curricular activities or university clubs you are part of have a commercial element, such as dealing with finances, business needs or customers/ competitors.
I wanted to work in a firm that had relationships with large, multinational clients who are negotiating a global economy, complex laws and challenging business environments. I was drawn to the international aspects of commercial law. I considered that in order to get the best training and become the best lawyer, I wanted to work in a firm, and for clients who were part of this global economy.
It is also worth considering if there are any significant cases/mergers/ insolvencies that have particularly influenced your decision to pursue a career in law. Reading the Economist or the FT is a good way to pick up on key commercial changes.
Why your English degree?
Again, this is a personal question. I knew that I wanted to do law and that the GDL was available. Therefore, I saw no need to study law at such an early stage. I also wanted to read a subject that I was passionate about. Studying a non-law degree is good talking point in interviews, for example if you had a particular interest you focussed your dissertation on.
In addition, when you make applications, you can make reference to the useful transferable skills – written communication, analytical skills, attention to detail, which are all important as a lawyer. However, it might also be interesting to consider what you enjoyed about English Literature and how this makes you a more well-rounded individual.
Although justifying a career is commercial law is an important part of an application/interview process, also consider whether you have gained commercial experience in general, not just legal experience. As a non-law graduate, employers will not expect you to have huge volumes of commercial legal experience. Try and get some other legal experience (e.g. volunteering in Citizens Advice Bureau), which can enhance other essential skills (client awareness, communication etc.) and demonstrate your enthusiasm to pursue a career in law.