The legal definitions are.
Driving. “Being at the controls of a vehicle for the purpose of controling its movement, wether it is moving or stationary with the engin running.”
Dept of Transport GV262.
Breaks. “A period of at least 15 minutes during which time a driver may not carry out any other work. Waiting time & time not devoted to driving spent in a vehicle in motion, a ferry, or a train shall not be regarded as other work.”
Article 7 E.C. 3820/85.
So you may claim a break when you are waiting, but not if you are doing any other work, i.e. helping to load / unload. strapping down, refueling, getting paper work signed etc.
You may also put the tacho on break for less than 15 minutes, its just that youi cannot claim it as a break from driving.
Also do not forget that when you have accumulated a legal 45 minutes the 4.5 hours starts afresh regardless of how long you have driven for.
When stuck in stationary traffic I normally turn off the engine & throw the switch to break, & have been known to take a 45 minute break doing so.