I am not sure that immigration controls are the answer. Criminals are able to trick and bully people into working in a sub culture of low pay, which falls below the radar of the authorities. Let's target that. As an island, Britain´s border is more difficult to get through, but it is still frequently breached. I would hazard a guess that Germany has a far bigger problem of workers being smuggled in to work for very low pay.
There are similarities between the exploitation of all these groups, although prostitution is the most tragic. When prostitutes are imported, they face the fear of both deportation and possible disgrace/reprisals against their families in their home country. They are easier to punish than than their exploiters.
Sadly, this is also true of many of the exploited builders, cleaners and crop pickers, who come in from Eastern Europe. If caught, they are likey to be fined and have their earnings stolen by the host country. The authorities frequently punish the workers rather than the sods, who exploit them by trafficking them or paying below market (or even legal) wage levels. Humane immigration controls are defensible, but harsh punishment of exploited people is not making the law either enforceable or respected. I am not sure what you mean by "better immigration controls", Sapper, but I hope it is something along those lines.
I am in agreement with the general drift of your comments. It is obvious that the further we push drug addicts and prostitutes outside the law, the more likely they are to fall prey to ever more ruthless predators.