True.
However, the guideline is pretty much de facto. You may have heard of the 'speed awareness course' that you have the option to do. The range of speed to be applicable for that (as stated on my councils website
http://www.sussexsaferroads.gov.uk/the-speed-awareness-course.html) is as follows:
36, 37, 38, 39, 40, 41 & 42 MPH in a 30 MPH Limit
47, 48, 49, 50, 51, 52 & 53 MPH in a 40 MPH Limit
58, 59, 60, 61, 62, 63 & 64 MPH in a 50 MPH Limit
69, 70, 71, 72, 73, 74 & 75 MPH in a 60 MPH Limit
80, 81, 82, 83, 84, 85 & 86 MPH in a 70 MPH Limit
So essentially it is >+10% + 3mph and then a 6mph range.
This most certainly suggests the de facto rule is just that. If you got 3 points for doing 31 or 32 in a 30 from a Gatso (static) camera, then you should go and buy a lottery ticket.