Like for the other transmissions, 4s derailleur, 6s BWR+derailleur, Brompton offer the choice between several sizes of chainring including a smaller 44t.

A long cage derailleur cannot maintain the chain tensionned when the rear triangle is folded (this is valid for the Brompton and also for the Birdy, the Birdy Touring is factory equipped with a long cage Deore rear derailleur)), this has been tried many times.
There is a need for an additional chain tensionner whatever the length of the derailleur cage.
The Ltwoo Fold B special derailleur has an integrated long tension arm similar to the one of the Brompton tensionner.