As you suggested Brucey, I have measured the rim width at the middle of the braking surface with about 80 PSI in the tyre, not without tyre, as I did previously. Taking measurements around the rim, there is a short sector that is 0.27 mm wider than the rest. This seems to be the problem.
I have already seen that the Genuine Brompton rim is no less than 1.4 mm thick all round the wheel, at the braking surface, which is worn, but nothing outrageous, I'd say. 1 mm would be a rim for the bin, I would say.
So that's all. Just to buy a new wheel now. Many thanks.