Do you have a source for this information? Unless they actually call the doctor to verify, as well as check on some kind of register (I'm sure GMC registration isn't enough here - any serious terrorists would know a friendly medic), it's all a bit meaningless, isn't it? If the needles were going to be used to hijack the plane, I'm sure the terrorists could make a convincing letter.

Frankly, either the needles are dangerous enough not to be allowed on board, in which case if they're really needed they should be kept in the hands of the crew until injection time; or they're not. I suspect the latter.

Come to think of it, do planes have needles or various standard sizes in their first aid kits?

The syringes will not be a problem at all. Nor would the small quantity of liquid.

How big are the needles? If they were standard insulin needles you'd have a hard job doing much of an injury unless you stabbed someone in the eye. But I imagine you mean something a bit bigger? 1.5 inches? 21-gauge?

I'm not claiming to know the answer to this, by the way. But whatever the actually BAA answer is, I bet it's non-sensical.