Just noticed this yesterday...

 

There's a 'Mode' button on the bottom of the cruise control stalk which toggles between setting the ACC speed and setting the speed limiter threshold. 

 

 

Only if you have both

If you don't have ACC it toggles between the normal Cruise Control and the Speed Limiter.  I hit it by accident some months ago having not seen the well hidden button!

 

I find the limiter very useful in average speed check zones; there are a couple on the A14 I hit and with the one over the Orwell Bridge at Ipswich at 60mph it's definitely worthwhile to avoid a ticket (as my colleague found out to his cost!)

I found this by chance too. Not sure ill ever use the limiter as acc is much better.