Where can I buy a briefcase in London?

Part of the London adventure is figuring out where in the city to buy whatever you need. You read the first installment of this drama during our first few weeks here as we searched for housewares. Now, I’m in need of a roller briefcase (despite the fact I just put one given to me in the Spring in storage in the US, angst!). Where to go?

For those of you who are familiar with London, let me know if you have suggestions.