This is not strictly true. Not having a go cause not only is it really quite confusing but there's lots of information out there which isn't really worded that well!
For example - shops selling 'real' supressors will say you need an FAC to buy cause they are technically firearms. This is technically true, sort of- but only when they are attached to a firearm. Shops will ask for proof the suppressor is on the FAC due to 1) target audience 2) arse covering.
The definition of a suppressor isn't really that great imo:
"any accessory to any such weapon, designed or adapted to diminish the noise or flash caused by firing the weapon"
So there's no distinction between a 'real' and a 'replica' - could be a real KAC, could be the TM mk23 one, could be a coke can - the crucial bit is if it diminishes the noise or flash of a weapon and you dont have it on the FAC you are in trouble!
For example - its fine for an air rifle (-12ft/lbs KE) or shotgun to have a suppressor. Anecdotally sellers will ask you what you intend to use it for - if you want to put in on a firearm they will ask for the deets of your FAC, if air rifle thats fine.
Additionally, some 'real' supressors will be 'proofed' - if your FAC specificies a proof and you wanted to replace it, you would have to ask permission to do so. If your suppressor is unproofed you can replace with another unproofed supressor.
The law is messy but it helps to know that it was written with poaching in mind - context is important.
In short a supressor in of itself is not illegal but if you also own a firearm i would keep them very, very seperate should it raise eyebrows at an inspection. And i probably wouldn't bother getting a real one - you can still get into legal difficulty without having broken a law.
Sorry for going off topic.