That's a good question... never really thought about that one..? I don't know to be honest! the standard UK is this:
Driving larger goods vehicles (LGV) on a car driving licence
The Maximum Authorised Mass (MAM) (total weight of the vehicle plus the maximum load it can carry) determines the driving licence entitlement needed. If the vehicle has a MAM between 3.5 tonnes and 7.5 tonnes then a category C1 licence is needed, however, if the (MAM) exceeds 7.5 tonnes then a category C licence is needed.
You may want to post this in the Cash in Transit section if you haven't already.
You can drive a van on a cat B car licence. As all the security vans I've seen are transit type vehicles I think it would be a safe bet to say you'll be okay with a standard car licence.
That said the weight restrictions on vehicles still apply as stated by CD above.
Without knowing what sort of armour is applied to a specific vehicle your question cannot be answered.
If you are thinking of driving a security van as a career I would advise you to contact the companies supplying those services and ask if they have any licence requirements as they will be able to give you a definitive answer.