And how do you stop them? The UK is an Island and therefore, as long as the FRench allow UK borders in FRance- we have some control. After Brexit, UK border controls will have to move back to Dover and other ports in the UK- and the FRench won't be very active or keen to stop those who want to cross.
In Europe, there are border posts on main roads- but there are 100000s of miles of ungarded fields and forest, mountains, etc- where you can walk or cycle without anyone ever noticing. I live on the Swiss/French border- the border post is manned 3 hrs per day at random- but anyone can walk anywhere in the woods either side from FRance, and then on to Germany- they could be in Southern Germany in a couple of days...
Shall we go all Trump and decide to put a massive fence all around the UK - and between each EU or European country...
Our little country has border with FRance, Italy, Germany, Austria, Lichtenstein. Now, how tall do you want that fence?
The UK will soon have no control as to who crosses the Channel at all until they get to UK.