Under ACPO guidelines prosecutions can take place at Speed limit (+10%) + 3mph so if you are doing 36mph in a 30mph you can be prosecuted whereas if you are doing 35.9 mph you probably won't be.

I say "probably" because speeding is an absolute offence which means you can be prosecuted for any speed in excess of the limit - it's just that they invariably apply the ACPO rule. If you are below Speed limit (+10%) + 3mph there is a very high likelihood I can persuade them to drop the case.

Honest John is correct. Speed Awareness Courses (SAC) are usually offered (when they are offered) at Speed limit (+10%) + 6mph so there is usually only a 3mph window.

In 30mph zones they are normally offered at 39mph or below.

Many areas offering SAC only offer them for specific limits (usually 30mph).

I am not quite sure why this is although the relative risks (of causing say, bodily injury or death) posed by someone doing 39mph in a 30mph built up area (and hence qualiflying for a course) and someone doing 77mph on the motorway (and being refused one) is an interesting actuarial question.

having recently attended one of these courses,I can assure the OP that he need not inform his insurance company. Before shelling out £70 on the course iI checked with the organisers and was told that there was no need whatsoever to inform my insurance. That is one of the attractions of attending these courses-you leave with a clean slate. By the way,the course was informative and useful. The irony is though is that I never break 30 and 40 limits,but on this occasion I was late for a train due to unexpected traffic conditions,so the holier than though brigade need not get too excited.