the ban focuses on what the government call "realistic imitation firearms" (RIFs) which they fail to define.
Airguns will not be included under these legislations because they are technically firearms.
it goes like this:
Looks like a gun, but isn't over 1.35J in power = RIF
doesn't look remotely like a gun, but over 1.35J in power = Firearm
doesn't look like a gun and isn't over 1.35J in power = IF.
the government specifies that a IF can only be classified as not being an RIF if a non-trained person can differentiate it from a real gun without inspection further then looking at it on a table.
therefore, if a member of public mistakes it for a firearm, then its an RIF, even if it doesn't fire anything.
for more info, visit the ABA website:
http://www.associationofbritishairsoft.org/