Using a screw on wide converter with a fast lens will result in a poor quality image. If you need to shoot wide angle images at large apertures like f/1.8 then your options are limited to something like the Sigma 20mm f/1.8 or Sigma 24 f/1.8 both of which are around £450.
Alternatively upgrading your camera to the original 5D would cost about the same, and act like a 0.625x wide converter that improves image quality!