My feet are firmly in the camp that say that if I am out on my Brompton and refused entry in a shop or establishment with my folded Brompton, I turn around and look elsewhere. This in itself is quite interesting. Recently (on rainy Sunday morning with no one about) I tried to enter a small supermarket outlet on Oxford Street to buy a bottle of water and was refused entry. Moments later I walked into a high-end pen shop on Bond Street to collect a fountain pen that was being serviced and they could not have been more accommodating.
What I am saying is that I don't really use or need a huge lock. My Brompton always comes with me and when I do leave it a few metres away from me, I keep a close eye on it, being ready to strike with the ferocity and speed of a coiled cobra should anyone get too close!!
Occasionally I do take a lock and for a few years I have been using a HIPLOK FLX. This is a very compact lock with a light and all integrated into a clip that can fasten to bags, coats, belts and jersey pockets. At 100 grams you almost forget it is there.
The lock has a light that pumps out 10 lumens and has a constant and flashing mode. The flashing mode lasts for hour and hours.
It is a combination lock, so no keys are needed. The retractable steel cable extends up to 1 metre and affords basic protection for low-risk areas. The cable is also coasted in a plastic cover so it won't scratch the paintwork.
I have used this lock on trains where my Brompton is sitting on a luggage rack and at times where I just want some peace of mind. It definitely isn't a lock that you could use to leave your beloved Brompton unattended but as a cafe lock it is very good.
The retail price of the HIPLOK FLX is £49.99 which I think is too much. Shopping around I paid £19.99 which for me was a price that works. This lock normally stays inside my saddle bag or is clipped to a pocket and complements the smaller is better vibe of the Brompton.