The abhorrent practice of female genital mutilation has been against the law in this country for over thirty years but the history of its prosecution or rather non prosecution is abysmal: a single case prosecuted and the defendant was acquitted. It has brought the law into disrepute. It has been treated with disdain. It is illegal to smoke in a car when there are child passengers. Being drunk in a pub is surprisingly illegal. These two activities are and were driven by public opinion. In the former to emphasise the dangers of so called passive smoking where there is some debate as to cause and effect and in the latter to satisfy Victorian morality activists of the evils of working class drunkenness. For different reasons prosecutions are virtually non existent. The Hunting Act 2004 was enacted by Labour under Tony Blair to appease his left wing. It was therefore a demonstration of power and intent rather than legislation to improve the well being of society. It was for the very mirror in reasoning that persuaded Theresa May to put in its election manifesto earlier this year that if the Tories won a majority she would allow a free vote to overturn that act. 61.8% of all organised hunters charged with Hunting Act offences have escaped conviction.86 of the 165 Hunting Act charges made were dropped either before or at trial [52%]. 40 of these related to the big Heythrop trial. Complete statistics can be accessed here.
There are other such laws that might be regarded similarly. So to answer the question at the beginning of this post my response is that if legislation is enacted the will and the means to prosecute it must be available for all the law enforcement bodies associated. Failing to do so is a luxury we cannot afford. It is tantamount to rule of the mob where the mob is the lobby group or groups with most to gain for their own sometimes nefarious purposes.