taken from the Firearms act

6.15.2 UK Specification Imitation Blank Firearms
Imitation and replica firearms designed and chambered to fire blank ammunition without a projectile have been legitimately available for sale in the UK without any requirement for firearm certification for many years. Typically such firearms will be realistic in appearance but constructed of base metals and alloys. The hot gases produced by firing a blank cartridge in the chamber would typically be vented through an aperture close to the chamber and set at an acute angle to the axis of the bore of the barrel. The barrel would usually be fully blocked. This configuration for a UK marketed blank firing replica is arrived at in order to comply with the Firearms Act 1982 which established the criteria for determining whether an imitation firearm is readily convertible (to fire a shot bullet or missile).