I think it is only Basic disclosure for airside pass. This will not show any spent convictions.
From my experience :
The last two I did were full disclosures. I.e showing spent convictions.
As stated it's not normally the airline - it's the airside pass issue, although it is becoming more common for airlines to do background chceks as well.
Most common is now the statement from local police for last 3-5 years saying you were well behaved. I don't know how it worked in the UK because the UK police just took care of it but I spent some time in the US and my understanding meant if you were charged etc but not proven guilty the police still declare this. Not a problem for me but there were other students around who ran into the law. Don't know how that affected the reference if at all though.
On the last one they actually provided the list of convictions which made you ineligible in a list. Unfortunately I no longer have the list but for I do remember that a Class A drug conviction and murder were two of the convictions which were on the list.