1. There is absolutely nothing in international law which stops us sending these people back to France the Home Office just does not have the balls to send them back.
https://en.m.wikipedia.org/wiki/Non-refoulement
2. Amusingly, as I've written before, the only powers the froggies have to stop these people, once they are at sea, is a law banning 'non sea worthy craft' venturing more than 50m offshore, which they brought in to stop people trying to cross the channel in such as bath tubs to get in the Guinness Book of Records.