I got caught doing 36 in a 30 in the Limehouse Link tunnel (by Canary Wharf) heading towards Tower Bridge, in my old family wagon Renault Espace.
Stupid mistake, i was concious of speed cameras as there's one on the way into the tunnel, and signs throughout - but a coach was sat on my arse so i stupidly sped up to create some distance. Totally missed the camera (busy looking in my rear-view) until i saw the entire tunnel light up with a flash.
Anyway - I was offered SAC, i've done it - the course is good for what it's worth, not as bad as you'd expect.
Long story short, most insurance companies don't care - however anything Admiral group related (Admiral, Elephant, Diamond, Bell) do specifically ask and do then increase your premium similar to that of a driver with 3 points. e.g. there's no benefit to taking the course.
I'm with Admiral, because as a central London dweller, they're one of the only insurers that will offer a competitive quote - they just don't seem to treat London in such a negative light as all other insurers. Even with SAC declared, my premium is cheaper than any non-admiral group firms can offer.
I've read there's no access to the database of SAC attendees for any public/private companies, so technicallyroom i should be able to tell admiral i've not done a SAC, and be safe from being discovered, though if directly asked and then lying, i suspect there's some legal problem there if it did ever come out.
Needless to say, as soon as i can depart from Admiral i will. Greenlight look good. If you take the points, companies can check if you have any. So whilst there's no immediate benefit to the SAC for my premium, in the long run there MAY be - whereas the points are a dead certain premium increase for at least 3 years.