ya, even here in the US passing using the shoulder (unless there's some detour set up etc) is always illegal.
Not to mention, iirc driving on the shoulder is reserved in the UK for emergency vehicles. so that they can still reach accidents etc in traffic jams
That's what the shoulder is supposed to be for in the US too. Also, for people with car problems to have a.. I'll reluctantly say, safe place to pull off. Always ticks me off when I have to come to a stop to make a Left hand turn and wait for oncoming traffic... some jerk will go around me on the shoulder.
We lost two police officers to people driving on the shoulders when they shouldn't. One had a person pulled over for speeding and the other was working an accident with injuries (ambulance was there, first response fire was there too). He'd gone around to get something from his trunk and a person trying to get around the "traffic jam" flew up, running smack into the back of his cruiser.