UMMMM
You can legally get a "short barrel rifle" Which is a long barrel pistol with a rearward extension to the stock, this is normally in the form of a counterweight to the long barrel, but I can not remember the source but local gunsmiths will tell you chapter and verse.
The point is that it is weapons designated as "handguns" which are illegal and air pistols are not included in that group but their power output is legaly restricted and it is by exceeding the power output that it then becomes classed as a hndgun and as such illegal. I repeat the key word is "handgun" and the deffinition is very exact because the idea is to prevent the use of easily concealed weapons by irresponsible people.
Target Bunny