While I don't have the CC on my bobber, (but seriously considering it), I'm thinking that you may be able to get the control that you're seeking, from the throttle.
When you're cruising with the CC on, and you want to add a little speed, you should be able to do so by opening the throttle to your desired higher speed, (which temporarily deactivates the CC), and then hitting the CC button once. It should then carry on at the newly set speed.
If you want to slow down a bit, you can turn the throttle forward (which also temporarily deactivates the CC), causing the bike to decelerate until you've reached your desired speed and then hit the CC button once to continue at the newly set speed.
Forgive me if I've got it wrong as I've not read the instructions yet, but after riding an HD Road King with CC for the last 8 years, which operates similar to this; it would only seem logical to me that Triumph would have set this up this way.
Simple yes, and you're quite right saying that they could have made this standard or as an option for a few extra pennies;
with you on that.