I have Dave's old de-cat sitting in my garage, which is basically a pipe with two flanges welded to it.
Me and Dave would not recommend using that as a De-cat. As there is no silencing in the pipe the exhaust note become's nearly unbearable. But if you like it really loud (i could hear Dave's car from over 100m away before) then go for it.
The car would not be technically not street legal as the exhaust is too loud but you most likely would get away with this on day to day driving but at MOT time you would need to put the standard Cat back in as the car would fail on both noise and emmisions.
Mike