The women do come from the EU even African ones they come into europe and are trafficked to the UK under an EU passport. That will stop from Dec along with their gangmasters. Illegal immigration will continue but at least we have control over legal immigration for the first time in 30 years. The slavery will continue in europe where its always been prostitution is much more widespread (even normal) in most of the EU which is fine but there is a very bad side to the trade.
The only immigration you'll control is from the EU but your criminals enslaving girls are from Pakistan, that wont change with Brexit, so stop lying to yourself, unless you are not aware