I went shopping at the Tesco's near to where I am staying today. I wanted to pick up a few supplies and so I pedaled Doris my Brompton M3L to the shops with the Brompton T Bag (by far my favorite bag in the Brompton range... Well, my favorite out of the three that I own - the B bag doesn't count of course since that is a bag to put the Brompton into when I travel. I own the T Bag (it used to be known as the touring pannier) and the C Bag).
I overloaded it slightly with milk, bread, 2L of Pepsi, and other bits and bobs. Regardless of the extra load (so much that I couldn't close the bag), it still handled like a dream since the bag mounts to the luggage block on the front of the bikes this keeps the center of gravity very low.
It is so convenient having the Brompton with me in London. As on previous trips, I ride it between meetings and appointments. I use it for sightseeing expeditions. And of course I also use it to run errands!