Could you focus in on specific practice areas? For example, Irwin Mitchell does a lot of medical negligence work and is well known for it. Is that something that interests you, and if so, why? Maybe you did a module on it or have some relevant work experience and liked the overlap between medicine/dispute resolution.
As you mentioned, IM are a full-service law firm. You could keep your answer wider here and explain why that interests you. As an idea, you might want to explore working for individuals and businesses in your training (then explain why).
However you answer it, what’s important is to think about why it actually interests you. Where possible, try to back up your reasons with your experience to help justify your interest.
Digging into your interests like this can be tough. What helps me is when I’m researching a firm, including its strategy, practice areas or recent news, I flag the parts where I hear myself saying ‘that’s pretty cool/interesting’ or I feel a greater sense of excitement. From there, is to dig into why I found it cool. Perhaps it’s working for interesting clients, or because I know I’ve enjoyed that specific practice area from XYZ work experience/module/seminar/volunteering in the past.