Does a downpipe require a sports cat or decat?

[Homer]

The reason I ask is that I asked a garage to quote me on how much they wanted to fit one- they have come back saying do I want one with a sports cat or de-cat. I thought a downpipe would be a swap and the rest of the exhaust would remain the same, am I missing something? Thanks for any advice

[MC71]

Cat is part of the downpipe. 

[Scottymon]

Yeah, the downpipe on these consists of a Pre-cat > Flexi > 800 cell (iirc) main cat >
A picture speaks a thousand words, so:

[Homer]

So what would the difference be between a sports cat and de-cat?

[Janner_Sy]

A Performance downpipe essentially removes the pre cat from the downpipe completely and replaces the stock restrictive CAT with  performance CAT .  This is still rod legal and MOT proven.

A Decat downpipe removes ALL CATs from the downpipe completely.  This is not road legal nor is it MOT ready.

[Homer]

So how do people get away with a decat during MOT time?

[MC71]

You don't, put your cat downpipe back on for the MOT, PITA.