Now 10% + 1, I was told on mine a few years back. So I set the cruise control for 10% - 1.
I have always been led to believe that the ACPO guideline was +10%, and an ex of mine, WAY up in the CPS and involved with complicated speed enforcement cases, never corrected me. I am 99% certain that my recollection comes from an ACPO spokesperson on R4, umpteen years ago.
Note though that it is only a guideline. You aren't going to get nicked for it, but 0.00001mph over the limit is illegal and nickable. There is, or at least was, one Midland police force that state categorically that they do/did NOT stick to ACPO guidelines, but said no more than that.
Speedometers NEVER read low, always high. I have checked mine over the years against speed warning cameras that flash your speed, or against a sat nav in more recent times. Until my current car they have indicated 10% high, but my 2015 Megane reads only 5% high.
I suspect that the "horrible" comment is nothing to do with speed, but death rates on smart motorways. For a quick, simple and precise summary, check iPlayer for PM this afternoon (R4 - starts 5.00pm?)