I am now going to write something most sacrilegious:
I really don't get the 'folder' thing.
I resisted MTBs, but then I got it and love it.
I resisted fixed-gear, but then got it and love it.
Brompton-type folders just leave me cold. I've tried to love them, but they do not touch my soul.
Luggage is great and bicycles are great, but I'm not sure about making a hybrid of those two things.
A Brompton is fun to ride, but not as much fun as a nice, steel road bike.
I giggle on the Paddington train as people clamber on between Moreton and Reading with their Bromptons... It's a expensive way to buy the right to clunk other passengers in the shin.
I lose time by having to go to the rear of the train and get my bike, but despite being a rat, it is lovely to ride. A Brompton is a thing of wonder, but it is not lovely to ride.
I used to use 4x4s in my work (L/Cruisers, Defenders and so on). They are fabulous off-road, but just a dull, unresponsive, thirsty, noisy drag on-road. I would never think of driving one on tarmac unless there was no choice.
Similarly, whilst I love luggage when its function is to be luggage, I would not think of going out for a ride on it. It is luggage.
My advice (which you will not regret following) is to get a ratbike and keep it at the station in good but inconspicuous condition. Keep two if you want; one at each end.
Buy a Brompton if you must, but it's like plating football with a rugby ball. Amusing in its own way, but slightly missing the point.
PS. Although this is my sincere opinion, I confess that it is written with a smile not entirely without a slightly wry quality.