Brompton owners

Sorry that's what I meant, I was planning on swapping one of the sprockets, just not sure what to go for.

Have thought about upgrading to a 6 speed BWR and to hell with it, but want to try the cheaper option first.

You can add an extra sprocket on the back wheel, fiddle with the gear adjuster, and get a third gear onto the 2 speed without having to upgrade the hub. Cost will be next to nothing, unless you damage the tensioner and assorted pieces in the meantime. I have a PDF file that describes the process if you PM me your email.