Its not a Section 5 Firearm. End of story.
A firearm is "a lethal barrelled weapon of any description from which any shot, bullet or other missile can be discharged".
Its not lethal hence its not a Firearm. Further there is case law:
In R v Bewley [2012] EWCA Crim 1457, the Court of Appeal held that if an imitation firearm is to be treated as a firearm to which section 1 of the Firearms Act 1968 applies, the prosecution must prove that it can be readily converted so that it can discharge a shot, bullet or other missile.
Hence, this implies that a realistic imitation firearm (ie an AEG or GBBR) per se does not discharge 'shot, bullet or other missile' even though a BB could be considered a missile by a normal person. The need for the prosecution to prove this is the crux of the matter.
So, back to point 1. Not a firearm.
Next.