I mean no crappy BWR hub too, a real 11s derailleur with a real cassette. And no triple crankset, another remnant of the past same as the 4s cogset and derailleur.
On the Birdy, the only constrain is to put the chain on the smallest cog but this is not due to the chain tensionner, its due to the shape of the rear swing arm.
The rear swing arm locking works exactly like on the Brompton: its the seatpost pushed down that locks the rear swing arm folded.
With your setup, I doubt that the chain remain tensionned with the chain on the 20t chainring+11t cog?