The improvements are mainly cosmetic, the bikes certainly look nicer now.
Brompton could have sold a brass washer kit for £1 to replace the nylon washers in older brakes. Actually functionally there's not really any difference with the rear brake other than with dual pivot it's much easier to set up and keep self-centering properly. In terms of performance the main change with the brakes is the pads but of course any sane person would have swapped them out anyway.
Real improvements would have been to use chromoly for the frame, giving the weight reduction of ti parts but with none of the cost, getting rid of the inherently flexy wedge stem design in favour of something suitable for folding bikes (a la Dahon). Or perhaps just a reduction in price along with not selling rebadged components at double their normal price (bottom bracket, chainset).
Oh, the rear frame clip is a big improvement, of course.