Just a couple of words of caution on the Tern front.
Bikes like the BYB may have normal derailleur, but they hang very close to the ground due to the small wheels and as such are subject to all kinds of muck getting thrown at them. Normal gear drivetrains on folders will need a lot of love to keep them in fine fettle and working order. My personal preference is for either a single speed like mine, or an internal geared hub. Tern have bikes with either 3 or 7 speed internal hubs.
One potential major issue with Tern is the weight limit on the frame. Without checking, I think it's 100kg. I had an ex colleague who was a tall and stocky built chap, with winter clothes and work bag he was close to a 100kg. In three years, he had three hinge joint failures, I've also heard anecdotally of a number of others as well. I'm not so big myself, so come in a fair few kilos under the limit and as such have had no problems at all over the last five years.
On a slight side note, like you, I have no need for a smaller fold on my regular train, it's purely to meet the requirement for a free ticket. However, sometimes the guards have a purge and get really pedantic, one lady received a fine for unfolding her bike as we were slowing down to come into town. He said being as it was unfolded it was now a regular bike and she needed a regular bike ticket. I really couldn't believe the guard and had to really bite my tongue before I also got my self into trouble.