Well, that was easy. Took some GP-1's off my mountain bike as I wanted to replace them with a bar end version and simply took a box knife and cut the Brompton grips off. Terrible grips and not worth saving IMO. Didn't bother to clean the goo off as it provided a tacky surface to help with the new grips (which easily slide on).
Measured to the brake lever housing, grabbed a hack saw with a fine tooth blade, and made a clean cut to each grip. A little sandpaper to clean up the edges, slipped the grips in place, and Bob's your uncle. I know a few have put GC-2's with the stubby bar ends on but I didn't think they'd work well for me and my hands.
It's a *significant* improvement and a much easier job than I feared it would be. Highly recommended.