That newflangled marathon-winter-without-the-studs doesn't appear to be available in Brompton sizes, unfortunately.
I've got non-plus Marathons on mine, after the untimely demise of the original Brompton greens. Seems okay for not-too-muddy off-road, as I'd expect for a Marathon, but I fitted them late in the year and they haven't had the thin-film-of-leaf-slime test[1].
[1] Which the 40-406 equivalent failed spectacularly on my tourer a couple of years ago, unceremoniously dumping me in a heap.