The signs are erected by religious zealots who think they can act as judge, jury and executioner. Clearly, they have no legal standing whatsoever. However, the message from the makers and distributors of the signs is crystal clear – Sharia is the preferred method of law enforcement in the UK.
It's a very short step from posters on telegraph poles to mob rule and vigilantism. Even though so called 'Sharia controlled areas' are legally illegitimate, the posters promote the idea that they are legitimate and as such send a very dangerous message to impressionable people. They imply that it is legitimate to create areas where the host country's rule of law can be either circumvented or entirely overruled.
Sharia law eh? Great if you happen to own a pair of testicles and worship Allah. Pretty poor if you fail to meet one of those criteria and even worse if your fail to meet them both.