The do check, airguns can be seized by the Police and tested, if it's producing more than 12FPE your guilty it's that simple.
It's an "absolute offence" which means that NO EXCUSES (eg it's new out of the box)are excepted, if it's over your done!! This conviction would get you a criminal record for a Fire Arms offence a serious matter.
It becomes complex because the conditions of test are not specified by law so any pellet can be used with the obvious nightmare results for PCP shooters.
The Police rarely wander about snapping up airguns generally your rifle would only be tested if your were in an inappropriate place with it or involved in some sort of criminality.
The result of this is that most UK based Airgunners are a bit neurotic about FPE.
Regards Ora
Rivington Riflemen's Website -
http://www.rivington-riflemen.org
My Website -
http://www.ora8i.rivington-riflemen.org