And who decides if it is unjust?
It's up to the individual to decide if a law is unjust. That doesn't mean that you can then flount that law without fear of reprisal, but it does mean you should act to have the law changed.
Personally I believe in living my life by what is right or wrong, rather than what is lawful or unlawful. In most cases the two coincide and all is good. I think it's wrong to kill someone or steal their stuff, and so does the law. I think it's wrong to drive at an unsafe speed, but to me, 90mph on the motorway, especially without much other traffic is not unsafe and therefore not wrong. My view differs from the law there, as it does where pot is concerned for example. (I haven't smoked pot in about 6 or 7 years by the way). Smoking weed is against the law, but it's not wrong, there is no moral reason not to and so i don't see why i shouldn't.
However, I am aware of what the laws in this country are, and that I am liable to get in trouble for doing things that break the law and i accept that, because I want to live here at the moment. I wonder which laws the Freemen would like to see upheld if someone harmed them in some way? Makes me think of the travellers on Dale Farm, who broke the law in one direction by building on land that they didn't have permission to build on and expected to be allowed to get away with it, then used the law in their defense to try and remain there without being evicted.
TL;DR
You might or might not agree with the law, and you might or might not obey the law, that is up to you. But while you live in the UK you are governed by the law. If you think a law is daft then break it, but you may have to face consequences. If you don't like it, work to change the law or go somewhere else.