First North Western considered ordering a build of new one-car DMUs two decades ago, when long-term passenger numbers were still in decline. They decided against it back then, and we got 2- and 3-car 175s instead. I'd be very surprised if the idea is seriously considered again, given the growth in passenger numbers we've had since, and the need to make space-intensive PRM provisions in whatever stock is built. The number of services that would be comfortably operated by a PRM-modifed 153 (and no more) is vanishingly low, and not worth bespoke provision. That mirrors Network Rail's conclusion in it's rolling stock strategy of a few years back; that rural rolling stock would continue to be add-on orders to, or cascades from, regional fleets.
A mix of 2- and 3-car units can give much the same flexibility in capacity for individual diagrams*, without the added expense of the extra hardware in the detachable single units. See the class 170, 172, 175, 195, and 197 for examples of a 2/3-car fleet, or the 171 and 196 for 2/4-car fleets.
*more capacity, when you consider the third carraige doesn't need two more cabs, its own wheelchair spaces and large toilet, if those are already provided in the other two carriages.
