There is no grey area about this matter. One cannot legally possess a semi auto air rifle that is capable of more than 12fpe, nor can one possess an air pistol that is capable of more than 6fpe (semi auto or not).
By the letter of the Law low power semi auto air weapons are not permitted. HOWEVER, it is acknowledged in Home Office Circular 68/97 that it was NOT Parliament's intention to capture low power air weapons in the 1997 amendment to the 1968 Firearms Act. This is further corroborated by the CPS guidance:
http://www.cps.gov.uk/legal/d_to_g/firearms/index.htmlI don't know how to post a *.pdf but if someone will guide me, I will post the HOC 68/97.
Further texts corroborating that low power SA air weapons are permitted include: -
12th Report of the Firearms Consultative Committee:
"9.19 The Home Office view that low-powered self-loading rifled air
guns are not subject to Section 5(1)(b) of the Firearms Act 1968 should
be confirmed at the first legislative opportunity. [Paragraph 11.26]"
HO Guide on Firearms Licensing Law March 2015, Paragraph 2.43.: -
"Air guns, air rifles and air pistols are exempt from the certification requirement if they are not of a type declared specially dangerous by the Firearms (Dangerous Air Weapons) Rules 1969, as amended by the 1993 Rules or the Firearms (Dangerous Air Weapons) (Scotland) Rules 1969, as amended by the Firearms (Dangerous Air Weapons) (Scotland) Amendment Rules 1993."