So if it doesn't pass an MOT its not road legal
Not passing an MOT and not being road legal are 2 different things. if your car fails the MOT then its not classed as roadworthy. This doesn't make the decats illegal.
There is no law that I know of that requires a road going car to have a cat fitted.
there was a law bought in that all cars made after "I think 2001" or round that time that all cars must be fitted with a Cat.
I did look into it a few weeks back due to putting them on the website, I uploaded some this week and have put 'not for road use' on them.
It is a grey area but if you were to be pulled by Vosa for an inspection and you had decats then you would be fined and ordered to remove them within 14 days.
So... It's illegal to drive a car without a valid MOT and Decats won't pass an MOT. So put it this way, driving a car with decats isn't road legal.
Like I say it's up to the end user but I personally wouldn't take the risk, although tons do.