It is an offence to make, import or sell a realistic replica firearm. A realistic replica is one where only and expert would know that it was not real. Remember that the Police Armed Response Group could attend and they will have to make split second decisions as to whether the firearm is real or not, and the consequences could be fatal. It is also an offence to sell an ordinary replica firearm to someone under the age of 18.
If you are a parent, never buy your child a BB or replica gun, or anything that has the appearance of a real firearm. You could put their life at risk and they could end up with a criminal record. If your child already has a BB or replica gun, make sure you are aware of the law, and monitor what your child does with the weapon.