First, look at what missing information you still need to know and ask about it. Then take some time to think about a good initial solution, but be open to new requirements developing, as well as taking in feedback from your interviewers. It's good to approach this as a team discussion that would happen during the course of working together.
It's clear StashAway is looking for candidates who are seriously interested in working there, and this isn't just a generic intro question. So it's best to do your homework on the company as you'll be asked what you know.