LOL, Scarry, do you rally think that it is reasonable to say that everyone who destroys their papers is automatically a threat? That just does not make sense. The majority of migrants are people fleeing war, persecution or poverty for a better life, not terrorists. Many may destroy papers and get fake ones in order to increase their chances to get asylum, not because they're terrorists, but because they're desperate people who'd do anything to get a better life for themselves and their family. I think most people would do the same in their situation. Not even UKBA assumes that all migrants are automatically a security threat, that's just bonkers.
And yes, you're right, there have been a few incidents of terrorists coming in posing as migrants, but they are very, very few. The majority of terror attacks have not happened this way. To say you'd close the Eurotunnel for this reason, or consider all migrants a threat for this reason, is as crazy as saying that you'd assume all school children were shop lifters because one in a thousand is, so forbid all school children from entering shops.
I am not being blasé about threat levels or terrorist threats, but I do think that it is important to keep in mind that most migrants are not terrorists but people in need of help, and to focus on what is the most logical and effective way of preventing terrorism, and I do not see that being shutting down the Euro Tunnel, or for that matter, shutting all migrants out of the UK!