Chelsea Physic Garden in Kensington and Chelsea

You're standing at the Chelsea Physic Garden, a walled garden just off the Thames. Take a look at those tall brick walls. They're not just there for privacy. They trap the heat, so it's noticeably milder inside than out on the street. That's how the garden can claim the largest fruiting olive tree in Britain, and the world's northernmost grapefruit growing outdoors.