If the Home Office, Police and Courts agreed with you there would be prosecutions and withdrawal of stock - I'll keep an ear open for the rush of RFDs taking them off the shelves.
Meanwhile please find below CPS guidance on deciding if something is a firearm and note that although "Lethality" is used in the description of "Firearms" what a court actually evaluates is whether it is capable of causing "more than trifling and trivial" injury and anything which is is therefore is a "firearm"
A firearm is "a lethal barrelled weapon of any description from which any shot, bullet or other missile can be discharged" (section 57 (1) Firearms Act 1968), it includes:
any prohibited weapon (see below in this guidance section 5 Firearms Act 1968), whether it is such a lethal weapon as aforesaid or not; and
any component part of such a lethal or prohibited weapon; and
any accessory to any such weapon designed or adapted to diminish the noise or flash caused by firing the weapon.
Lethality is a complex issue and although case law exists (Moore v Gooderham [1960] 3 All E.R. 575), only a court can decide whether any particular weapon is capable of causing "more than trifling and trivial" injury and is therefore is a "firearm" for the purposes of the Acts.
A well placed .177 dart would be a capable of causing a "more than trifling and trivial" injury and that this capability alone would keep any .177 gun firmly in the "Airgun" classification