Bromptons aren't desperately heavy as a bike, although as a piece of hand luggage you often wish they could be lighter (hint: get Eazy-wheels and don't carry it unless you really have to). The main advantage of the Ti rear frame to me is simply that it doesn't rust, whereas the steel ones are gradually eaten away from the inside of the open tubes.
There are other aspects of the design that they could and should address before thinking about a titanium frame: the crude and expensive-to-fix rear hinge design, the head tube stretching due to lack of reinforcing rings, the soft paint, the highly variable wheelbuild, the rim tape that is too wide for the well of the rim and makes fitting some tyres impossible, the fact that no-one likes the standard gearing, the incredible faff of removing the rear wheel (why can't the chain tensioner have its own boss?) etc. Most of this would cost pennies to address.