There's nothing wrong with a 'thank you' to people who do what the Highway Code says they should be doing anyway.
As an HGV driver myself I know what it's like to try and safely pass a cyclist. A fully laden truck is frequently slower than an accelerating cyclist! In a town, I generally don't bother overtaking as it only means getting to the next lights/junction to be passed by the cyclist again. On the open road, I wait until there's a decent space to overtake. Often, a cyclist will pull over for a few seconds to let me past. As they can't see a wave, I give them a toot of thanks on the airhorns.
