As has been said he hasn't broken the law as it will still be classified as an air rifle without it being re-designated by a RFD. Also the barrel length restrictions only apply to licensed firearms and not off the shelf, sub 12ft/lb air rifles.
If you were to do this though it really does defeat the purpose of the initial rifle. The barrel length provided on a standard rifle is normally optimised with the rifling to allow the best power curve over time. By shortening the barrel thus reducing the length and rifle it becomes even more inaccurate and ineffective. Just stick with a normal designed carbine or pistol (although I would not advocate using a pistol for any hunting or culling)
If you were to do this though it really does defeat the purpose of the initial rifle. The barrel length provided on a standard rifle is normally optimised with the rifling to allow the best power curve over time. By shortening the barrel thus reducing the length and rifle it becomes even more inaccurate and ineffective. Just stick with a normal designed carbine or pistol (although I would not advocate using a pistol for any hunting or culling)