For the Latham open day application the first question is 'What area of law are you interested in learning more about, and why?' Would it be okay to say employment law - is this something the firm has a focus on?
While Latham does have an employment & benefits practice in London, it is definitely one of their smaller ones. As with other top US firms, its role is probably to generally service clients on the employment-law elements of a deal the firm's transactional departments leads on, rather than working on independent employment law-focused mandates. Thus, Latham is not really focused on employment, and is not particularly known for its expertise in this area - if you check the latest Chambers UK rankings here, you'll see Latham has not been ranked for its employment practice (
https://chambers.com/legal-rankings/employment-uk-2:23:224:1). On a side note, if you are really interested in employment law and want to work with the best law in this field, Chambers ranking are a really good guide to finding the right firms to apply for.
For Latham, it's London focus (and top expertise) is on private equity, banking & finance, capital markets, and corporate M&A. Latham also announced it would seek to grow its litigation practice, but there is still some way to go before it will account for a substantial portion of the office's revenue and headcount. If you want to maximize your chances of succeeding in your application, even if you are somewhat more interested in employment law, consider choosing one of the transactional practices. The caveat to that is that you should only talk about a practice you have a genuine interest in - there's no much point in making applications for firms which focus on work you would not want to do, and recruiters are likely to notice if you are feigning interest anyways.
However, if you do not presently have a genuine interest in any of those practice areas, I would not completely rule out applying to a firm like Latham. In my experience, many people (myself included) do not know from the start what parts of commercial law they could be attracted to. What tends to happen instead is that as you learn about different practice areas, you will slowly start developing an interest in some of them. Thus, if you do not currently have an interest in many transactional practices, and if you have not already done so, I'd encourage you to read some introductory materials on M&A/finance deals before deciding on whether to pursue an application with Latham.