srw
It's a bit more complicated than that...
Crikey. I've always stood up on the pedals on my Brompton and have never ever had a problem, with or without front luggage. And I've always been a lot heavier than 60kg - or even 70kg. Big holes are a problem (my one solo off in recent years was going very slowly on a downhill on the Brompton when I failed to notice a large pothole), but they're a problem whether you're standing up or sitting down. And I've never had an anti-fold clip - and if I did I'd whip it off PDQ.