Not true. There is no specific legislation that prevents their use on the road.
You're not allowed to modify your exhaust with the specific intention of making it louder under the Construction & Use Regs, however there's nothing that says you must not remove your cats. It is highly unlikely (read: impossible) that you'll pass an MOT with decats on, and it's illegal not to have an MOT, however that does not mean that you can't alter your car as soon as you have the certificate in hand.
I know exactly what you're trying to get at with your post and generally you're spot-on, I just feel it's important not to propagate the incorrect statement that decats are illegal.
If it were me, given the hit and miss nature of hi-flo cats I'd either decat and replace with the cats once a year, or stick with the stock cats.