No they won't but their vehicle inspectors will... actually, knowing several coppers, some of them WILL crawl under a car. Traffic cops know what they're looking for and a decat isn't hard to find. So yes if your cat is knackered, but if not you might have difficulty claiming that when there's a blatant decat pipe.
Thing is, you've gotta remember that as young male car drivers, we're public enemy number one and public enemy number one A star if you drive a modded car. So they will blatantly have clue about who has a decat. My mate got stopped in his scooby, they found the decat pipe and they asked him to sort it out as they 'suspected' it wouldn't pass its emissions test.
If you do get stopped by coppers with a vehicle inspection team and they check your emissions cos they suspect you have a decat, they WILL impound your car as technically it is not in a legal roadworthy condition. You would be amazed at how far coppers will go to get someone busted... did you hear about the JCB driver in Bournemouth who tooted his horn at a mobile speed camera? He actually gave them a thumbs up sign but seven coppers in a wagon followed him a home and gave him a £30 ticket for using his horn incorrectly. Says it all...