They become economic migrants at that point. They avoid a "safe" country in the hope of reaching one which suits them better in terms of economy/opportunities. The issue of migrants trying to illegally enter the UK via Calais has existed for decades at this point, any improvement to the conditions in Calais would encourage more people to try and make the journey so nothing is done for fear of turning a bad situation into an even worse one.

The UK public saw what happened when Mrs Merkel said there was no limits on the number of refugee's Germany would take, they are not in the mood for their politicians to make a similar mistake.