Firstly, legally, an air rifle is limited to 12 ft.lb before it becomes a section 1 firearm. An air pistol can only be 6 ft.lb. Make no mistake, if you are caught with ANY weapon that's not held on an FAC that produces more than 12 ft.lb, with ANY projectile that they can stuff down it, then you WILL end up with a criminal record and you WILL forfeit the gun.
Now, for a weapon to be considered "airsoft" (i.e. a toy, not a "lethally barreled weapon for the purposes of the firearms act) Home Office guidance sets out a muzzle energy below 1 joule, so about 330fps with a .2g projectile.
In theory, anything above this level is considered a "lethal barreled weapon" and falls under all the provisions of the firearms act 1968, as amended.
Practically all this means is that any weapon which is capable of firing more than one projectile without repeated pressure on the trigger (i.e. fully auto) is a prohibited weapon under section 5 of the act. This is why single shot, and semi auto guns are often seen with higher energy levels.
Now, this isn't actually included in settled law, only in guidance, but it would seem silly to risk the criminal record and seizure of guns that might result from a prosecution for having a S5 firearm without a certificate.
For one joule, with a .2g bb, your velocity will be 328.08 fps.
To hit the 12 ft.lb legal limit before a firearms certificate is required, you would need to propel the .2g bb at 1323.4 fps, which is going to be tricky with any of the standard mechanisms used in bb guns.