Well, as previously mentioned, I thought Germany was 3mm limit....
I can see your point Rob - but then again if you checked your tyres and you were already on or below the TWI (you'd not know if you were below unless you measured it, because as the tyre tread wears lower the TWI will wear down as well) you'd already be breaking the law.....
It's still an intreguing (sp?) question though... I just know I'm going to change my tyres before they get that low - just this one caught me off guard.