It is also policy issue. From what I’ve seen and read, many of these people have already lived and worked illegally in UK, and that is why they are trying to get there despite being already in France.
The Czech Republic has the following policy: once a refugee that is already registered under Dublin is caught in the Czech Republic, they get judicial order to leave the country back to the EU country where they were first registered. (The transfer may be paid by the state)
When the refugee gets caught the second time, it is considered breach of judicial order which is a criminal offense. That leads to criminal conviction (often with remand during the proceedings in standard prison) which in case of foreigners in 99% cases includes expulsion, so they get put on a plane back to the country of origin (imagine that after spending all your livelihood and risking your life getting to EU).
Nobody wants to be caught the second time in my country.
Fun fact: Randy Blythe of Lamb of God was in jail cell with two Mongolians facing breach of judicial order charges and waiting to be convicted and put on plain home.