That Brompton paint will work but I would personally just pop down the hardware shop and buy something like Hammerite or Japlac. Much cheaper.
The most important point is to fully degrease the parts of the frame you want to touch up as any oil, grease, wax or silicone will play havoc with fresh paint. A motor factors or
Halfords should be able to provide "panel wipes" for the job, otherwise, a clean cloth with some paint thinners on it would do.
Edit: If rust is already present, a little dab of Hammerite Kurust or Jenolite may be worthwhile before the paint.