The Schengen regime allows freedom of movement within the EU and that's the main reason why people migrated or were trafficked to northern France so easily for years. After that, well it's just a short ride. This was all encouraged more by Mutti Merkel in 2015 and never slowed down. Now the EU is seeing so much kickback from major member states that it's finally reversing its ideas on immigration.
The UK immediately lost full sovereignty in 1973, when it joined the EEC (Treaty of Rome), due to the obligations to fulfil the requirements of EEC/EC/EU Treaties and law, which have primacy in UK law. Successive treaties were not put to the people by way of referenda but these eroded away more and more sovereignty. The UK was about to lose even more but Brexit slowed down the EU steamroller in many ways.