And who will carry out the enforcement?
The police are woefully inadequate and will be focussed on fighting the war againts terrorism as that's the greatest risk to the population. Prostitution is low level stuff. They cant even find resources to investigate burgulary, with is far more intrusive to the public.
Despite numbers of police rising in past years, all police forces mow face financial cutbacks, and as most of us will have noticed, there are few, if any, police patrols on major A roads and motorways. So, if it became illegal, how would it be policed? Have a plod outside every hotel, or whore's house?