http://www.cleveland.police.uk/young-people/The-Law.aspx
Q. Is it illegal to own a BB gun?
A. No, but only under certain, strict circumstances:
You must be 18 years old or over and be a member of a soft-air club to buy and possess a realistic BB gun.
If you are not a member of a soft-air club, but still over 18 years old, you can only purchase a non-realistic BB gun, which will be manufactured in an obscure colour (pink, yellow, etc.).
If you are under 18 years old, you can only possess a BB gun on private land.
BB guns (which fire ball bearings or plastic pellets) are often played with as toys but remember they are not toys and they can be very dangerous and, potentially, could seriously injure or kill someone.
You must remember that a BB gun is considered to be an imitation firearm, which is defined as:
"any thing which has the appearance of being a firearm whether or not it is capable of discharging any shot, bullet or missile”.
It is illegal to carry an imitation firearm in public and the consequences could be very serious. If you carry a BB gun in a public place, you are potentially risking your life if someone sees it and calls the police.
When the police arrive, they will not know for sure that it is a BB gun. Even trained police officers have great difficulty distinguishing these guns from actual firearms and have to act as if they are real.
There is a real risk of you being shot by armed response officers who are unable to establish the exact nature of the weapon.
If you own a BB gun, the safest thing to do is to hand it in at your nearest police station.