Undertaking… is it legal? well its not illegal but can be dangerous. Never really done it in trucks except odd occasion when for outright safety reason you must do it as long as you KNOW what you can do. Nowadays I avoid it because us truckers although we love racing these elephants around must stay Professional 
In my car its a different story though, too many car drivers and a few van drivers take the ■■■■ by hogging lanes and or just dawdling around don’t pay attention to there driving or lane discipline enough because they don’t care and don’t stay alert. So sometimes I will undertake in my CAR safely and quickly 200bhp Ford Smax TDI, remap soon 
because I don’t hang around and dawdle on the roads. Sorry but if your taking ■■■■ on the road and you creating a nuisance of your self you will get dealt with. Advanced defensive driving safely… Motorways and Dual Carriageways mainly. ADDS