The UK and French governments are facing a crisis at their mutual border which is of their own making. This is because stopping refugees from moving does not solve refugee migration.
Refugee migration is caused by events such as wars, and so in itself it cannot be solved by closing borders to refugees. The refugees still exist no matter how many doors slam shut to them. Imagining them away by keeping them out of your own country is therefore only ever a displacement strategy – never a sustainable solution.
Most people don’t actually get very far. A full 85% of the world’s refugees are in countries neighbouring the one they fled, so the burden of hosting them mainly falls on less wealthy countries. But some do travel further afield, and saying they are economic migrants or blocking their passage will not make them disappear. It will instead almost always lead to more deaths.