It's an "It Luggage" model 12-0733-09, quite similar to this one: PORTO - STYLISH AND DURABLE EXPANDER | IT Luggage

I got it at a local department store. Basically I rode the Brompton to the store, folded it, and asked the salespeople to show me suitcases where it would fit.

There was a really good looking Victorinox suitcase where it fits, and it seemed really well made, but it was too expensive. Let's see if I regret going for the cheaper one after my trip