It's not that long since they moved into a nice new factory, which I bet is running at capacity right now! It's not clear how long the current peak in demand might last, but in the current state of economic uncertainty I doubt they have any plans to expand just now.
Even if they did, Brompton frames are brazed without lugs, a process that isn't commonly used in cycle manufacture. They train their own operatives in house but I believe it takes years to achieve full competence. The bikes themselves include an awful lot of components which are peculiar to Bromptons so even if they could increase frame production they could still have supply chain problems.
It's as if they have reached the small manufacturers equivalent of the "Glass Cieling" Extremely difficult to progress further without help, probably of a sort you don't really want.
For discussions about bikes and equipment.
IIRC they expressly built the Greenford factory so that they were only at about 60% capacity when they moved in; you'd have thought they'd have expanded this as much as possible, especially as they seem to have been running at about two months lead time for non-standard bikes even before Covid.