Legally speaking - I don't know, and I am sorry I can't give a citation to a specific law that will clear this up.
Practically speaking - The difference between a fixed blade and a folder of equivilent blade shape/size is more about ease of presentation, not effectivness. A modern, good quality folder is more than solid enough to use to end a violent attack. In a self defense situation slashes against the attacker's arms & legs will stop their ability to attack you faster than a stab. Stab wounds tend to be more lethal eventually, but a slash on an arm that damages a nerve and prevents the attacker from holding a weapon will end the attack faster. From that point it is a matter of which is faster to deploy - a folder or fixed blade.
I realize that none of this answers your question, but it may give you something to consider when training and deciding what to carry.