You are clearly not fully understanding FoM and mixing it up with passport free travel:
1/ FoM is a citizen right of EU citizens only which allows them to move to and settle in any EU country.
2/ Passport free travel is something which comes with the Schengen Area and basically means all people within the area can travel from one Schengen country to another without being checked at the border.
My mistake, I assumed the bulk of the immigrants were illegals who arrived on the boats picked up in the Med and were detained and or housed in camps by the nation in which they landed, was unaware that a lot of them had entered the EU with Schengen Visa.
In the assumption that most migrants to Europe are indeed picked from their boats in the Med and do not make it into Europe completely undetected, they are mostly not detained in camps like you think they are!
Yes, there are a few of those migrant sites in Greece and the south of Italy for purely administrative reasons (because of the massive influx of immigrants), but other than there, the general rule throughout Europe is that migrants are simply registered, their asylum request processed and then sent off to some social housing and humanitarian assistance providers further inland prending a formal decision.
It is during this procedure that they can continue freely along their path to their final destination, which is often the UK, as in many cases the migrants aren't very interested in staying in the first safe place where they made the demand for asylum (Greece, south of Italy, etc) of course.
Now, currently the French are so kind to stop them at the channel, but as Macron has already pointed out quite a few times, this kindness might change in future and that would be a HUGE problem to those in Britain claiming to be taking back control of the British borders, because let's face it: you can't effectively control a border if the neighbouring country is not willing to keep people away from that border in the first place!
No sea is ever going to be deep enough, no dessert wide enough, no wall high enough unless you turn it into a Berlin Wall type of border, that is, including mine fields and all other niceties.
It's why the EU has a deal with Turkey in place to keep potential migrants away from its shores and why even bigmouth Trump has now struck a very similar deal with Mexico to have them effectively do the same kind of work for him well before migrants to the USA ever show up at his border wall, so it's completely insane to believe a channel crossing of just a couple of miles with a few patrol vessels in it could effecitvely allow Great Britain to take control of its borders: in reality the door to the UK is wide open by geographical design, and the only thing preventing the UK to be overrun is the fact that fellow EU countries now make sure very few people manage to make it through the door for you!
The UK is going to have to rely heavily on the French, the Belgians, the Dutch and others in the Schengen area to keep migrants away from its borders in future too, similar to how they themselves rely on others to do the same thing at their borders too: Britain really is no exception to this, Island or not: the Med and the Sonoran dessert are far bigger obstacles than the Channel, I should think?
And let's not get fooled: Britain is going to start to pay the EU for that service in future once it's out and thus no longer part of the club which defends its member's interests, of course!