I dont believe police should be able to do you for careless driving as it is down to the cop there and then so just down to his personal preferences there will be no black and white with it, it will be all a grey mess.
as for undertaking.... im all for it i dont see how it is any worse than over taking someone. I admit if there is a car in the middle lane and refusing to move over despite the inside lane being clear for the next 2miles i will undertake them (i see it as if i have the space to safetly undertake them and no have to pull out straight away cutting them up then they are in the wrong not me), they are cause more of a danger by forcing everyone into one lane than i am undertaking them and yes if they pulled in on me it would be classed as my fault but 1 - if they werent in the wrong lane to begin with i wouldnt of been there 2 - they should be using their mirrors/checking their blind spot before making any move so they should have spotted me.
middle lane drivers and not using indicators are the only thing that gives me road rage.
tbf on a whole the legal system/punishment system in this country is wrong.