Bart pointed me at this Brompton innovation:

After many years of denying customers an anti-flop clip, Brompton has relented and in typical fashion has introduced a small engineering jewel. This is from their memo:

Why did we do it? Over the years, countless Brompton owners have come to appreciate the way the Brompton’s rear wheel folds under, the ease of parking generally far outweighing any inconvenience in carrying the unfolded bike. However, some consumers remain unconvinced by a bike that is not “fixed” under them as they ride, while others, who regularly carry luggage on the rear rack, would benefit from having their loads secured to the mainframe when lifting the bike over obstacles or up stairs.
The RFC, described as “a development in the best Brompton tradition”, aims to offer the best of both worlds – the rear frame can be fixed or allowed to fold under as the rider chooses; the additional weight of the latching mechanism is almost entirely offset by the new lighter seat clamp, the whole package being just 1g heavier than a pre-July Brompton.

I don’t feel the need, but then a lot of the joy of the Brompton for me is that I can pick it up one-handed, and if I have to manhandle it then my arms are long enough to grab the rear rack.

But I do want to upgrade to the pannier bag, reusing my existing C-bag frame…