It's a flawed concept stemming from an inaccurate and incomplete understanding of the law and our legal system.
However if "Freemen" really don't want to accept any of the rules that govern the UK then I can only suggest they find somewhere else to live, you do not get to elect which laws apply to you and the claims about requiring consent to be subject to the laws is a complete non-starter. If you commit a crime then claiming that the police and the courts have no jurisdiction over you holds about the same weight as Slobodan Milošević claiming not to recognise the jurisdiction of the International Criminal Tribunal for the Former Yugoslavia - the claim is all well and good but it'll be ignored as it has no legal weight. However it appears that most "Freemen" want most of the benefits of living in our society but don't want the accompanying responsibilities that come with this, including the laws and taxation which they happen to disagree with.