They reckoned he was a **** because (a) the police were very visible, (b) he was on the slip road exiting the motorway and approaching a roundabout and therefore slowing down, (c) by his own admission he saw the police late and "stood on the brakes", (d) it was rush hour and there was a lot of traffic.
He was flashed at 138kph, but his speed would have been a lot higher if he hadn't been braking.
The "****" wasn't necessarily directed at him, or anyone else, who chooses to drive fast. Why, even police chiefs do that.
The feeling was that it was inappropriate in those circumstances and at that speed he should have been a lot more attentive -- the police were very visible, and if he didn't see them, how many other things did he not see?
In short, he would have done better not to brag about it.