For a definitive answer you will need to ask CCW, however I would expect the legal situation to be fairly clear cut.
SSSI's are designated on a map (topographically) and that is the only way they can be if they legally exclude certain actions (PDO's). If you are a land owner and have an SSSI on your land, you will have been told by CCW where the boundaries are and what operations are prohibited without CCW's consent.
If subsequently a cave is extended, how does the land owner know to extend the boundaries, how far should they be extended, does the possible extension involve just land over a cave, over the possible/probable extension, over the area of catchment, etc. These things can only be determined once a cave has been extended.
From a legal perspective the law can only deal with absolutes. SSSI's have to be scheduled through quite a complex procedure to ensure any infringements can be prosecuted through the courts. If a site needs extending then this process will have to be repeated for the extension, notification of the proposed extension, the opportunity to object, the final designation and notification of PDO's, etc.
This takes a long time and in the meantime the SSSI has not been extended.
If you look at the designation of the Priddy caves there is special dispensation in the designation for actions normally carried out by cavers. The Priddy SSSI's also have a large list of PDO's and they were originally applied to the whole area of land that overlay a cave but afterwards English Nature (as it was then) gave permission for all actions listed except quarrying.
It is fine to be seeking opinion from the caving community, but the only definitive answer can come from CCW.