scaryof course I understand what you implied, I just wanted you to explain it properly, as I suspected the argument, spelled out, would not make sense. Why would the tunnel be damaged by the French letting migrant jump on vans and travel to the UK? Migration has never structurally damaged the tunnel. Or do you mean that the mere fact the migrants are there increases risks of a terror attack, which would damage the tunnel? Of course, all major traffic points are potential terrorism targets, but migrants travelling though them does not, in any intelligence service's estimation, make them a more likely terrorist target. There has been concerns about terrorist potentially hiding among the migrant flows, and this way getting into the UK/Europe, but this is a different issue. Does not make the tunnel itself a more likely target.
Anyway, the fear of terrorist coming in with migrant groups is vastly exaggerated, as most terror incidents in recent years have been by so-called home-grown terrorists. But if you do want to talk about how to stop international terrorists from entering the UK, then the way to stop them is anyway never going to be mainly with increased border control, as most terrorists will have very good fake papers etc. The way to stop terrorism is mainly done by intelligence and intelligence sharing among countries.