As above really. The downpipe from the manifold has a precat at the top at the flange and there's the main cat at the other end of the pipe which will join to your Remus pipe. To remove the precat is okay. You may or may not need an O2 simulator. I've got one fitted but I know that there are some others who have not.
The main cat will need to be cut out if you are thinking of fitting a 200 cell sport cat. The new 200 cell will need to be welded in.
If you are de-catting, the main cat gets removed and a pipe welded. The car will NOT pass an MOT without the main cat.