Answer
This will depend on the circumstances.
It is an offence to possess an imitation firearm in a public place unless the person can prove they have lawful authority or a reasonable excuse. It is also an offence to manufacture, import or sell a realistic imitation firearm, or to modify an imitation firearm so that it becomes a realistic imitation firearm. There are some exceptions, for example, weapons used in historical re-enactments, theatrical productions and film making.
It is also an offence to possess an article capable of being used to convert an imitation firearm, where the person intends to use the article (whether by itself or with other articles) to convert such a firearm. Registered firearms dealers are exempt from this offence.
It is an offence to sell an imitation firearm to a person under the age of 18, or for a person under 18 to purchase one.
An imitation firearm has been defined as 'anything, which has the appearance of being a firearm whether or not it is capable of discharging any shot, bullet, or other missile' so could include toy guns. Many imitation weapons are very realistic and until the weapon has been seized, it is difficult to ascertain whether it is genuine or not.
All calls to police involving firearms are treated as if it is a genuine firearm so be aware that if you do wave an imitation firearm around you could find yourself surrounded by firearms officers pointing real weapons at you.