With limited exceptions (such as the ban on using a hand-held mobile phone while driving) there are very few 'specific' motoring laws in the UK. (i.e. laws which prohibit particular actions).
For example, there is no law that states that you can't drive along the right-hand side of a two-way street. If a police officer sees you doing so he (and the Crown Prosecution Service) needs to decide whether your actions amount to 'driving without due care and attention', 'careless driving', 'dangerous driving' or any other relevant offence. If you were driving a milk float along the right-hand side of a well-lit, deserted road at 5am he would almost certainly come to the conclusion that your actions were not illegal.
Similarly, if you're approaching a motorway exit, and pass on the near-side of another vehicle, a police officer can't automatically decide "that's illegal" (since there is no specific law banning drivers from 'undertaking', irrespective of where they are on the road). If, when you did so, you and the other driver were clearly driving within designated lanes, marked for specific routes, you wouldn't be guilty of any offence. However if the lanes were not specifically marked for particular routes, you'd need to exercise extreme care when passing to the left of the other driver (and possibly avoid doing so). Otherwise the police (and the CPS) might judge that your actions amounted to 'careless driving'.
Chris