I had a shop assistant complain to me that "there's a bike rack outside, can't you use that?" when I took my folded Brompton inside.
The shop in question? Evans Cycles. Which is full of bikes. Including Bromptons. Admittedly the ones on display weren't dribbling muddy water everywhere, which mine was.
When I bough my Brompton i was told by the shop that you do not need a lock for a Brompton, as there is no point in locking it up.
In London it will disappear within minutes, always take it with you.
(I do actually have a lock, I only use it when leaving the bike in secure places like cloakrooms and locked bike cages, I would never leave a Brompton unattended locked on the street)