Is there any way I can get this prohibition lifted early?
Were you an MP I wonder? The main reason for the Firearms Act prohibiting people who have served jail sentences in excess of three months from possessing a firearm (which includes an air gun) is to prevent weapons from falling into unsuitable hands.
It is not a form of supplementary punishment and the Act clearly allows for redemption in suitable cases.
You can apply to the Crown Court (Sheriff if you live north of the Border) for the prohibition to be lifted.
Provided you can satisfy the court that you do not present a danger to public safety or the peace it is likely they will lift the prohibition.
This applies to both five year and life prohibitions.