Threat of terrorists with Nukes

Does anyone here feel the threat posed by terrorists with Nukes or for that matter Chemial weapons to be underplayed?

We all know that Al Queda would love nothing more other than to sneak a Nuke into the US or Britain and place it in a van parked outside Downing Street or outside the Whitehouse lawn.

Question is could they do it? Are the respective nations security forces doing their best to make sure it doesn’t happen?

I feel that its a question of “When” rather than “If”. Sadly, the US will be the target as it would send a greater signal to the West. I can see it happening in my lifetime without doubt.