What is sure is that currently, Brompton factory has no in house aluminum frame manufacturing capabilities (nor in house aluminum frame design knowledge), unless they build up a brand new production unit for aluminum frames (welded, hydroformed or any other frame building) they will have to sub-contract the manufacturing of an aluminum Brompton frame just like they do for the frame of the T-line and titanium parts of the P-line what will increase the costs and be a problem to sell a cheaper lightweight Brompton.

If they sub-contract outside UK, their marketing image of a British bike will be lost.

For the weight, moving to aluminum doesn't mean that the bike will weight less, the Dahon Curl (the most similar aluminum folding bike) doesn't weight less.

Making a really lightweight bike like the T-line, whatever the material used for the frame, costs more money and end up eventually in an expensive bike.