It only takes a tiny vial
...and growth medium. All throughout the '90s, UNSCOM found Iraqis hiding and/or importing growth medium by the ton.
It's correct that finding this stuff does not, by itself, justify a pre-emptive war. What justifies a pre-emptive war are the NBC weapons combined with the extensive and ongoing links between Iraqi "diplomats" and al Qaeda operators.
I'll bet that the murder of Police Constable Oake the subsequent revelation of al Qaeda's ricin ring in Western Europe was the final straw as far as Tony Blair was concerned.