I've just has my car mot'd which it pasted

So basically my car is uk model with the jdm engine, at the moment I believe the exhaust setup is ukdm, the cat measures 350mm which I believe is uk?
I can get hold of a jdm decat pipe which measures aprox 270mm from looking at a picture.
So what I need to know since I know need a down pipe is what to get if I wanted to fit the de at also.
I've read a thread on here about the different lengths etc but still I'm none the wiser as so many people seem to have different opinions
I also have at home a cat back mongoose system which came off a jdm car.
So help would be great

Ps the actual manifold is a Megan racing item however the down pipe section just looked standard.