Yes i do know of this mega cliff of which you speak. No ther are no lower offs at the top of the cliff. I did try to self belay toprope the route on the the left hand end of the cave about 22ish. have been there about three times (girlfriends parents live 10 minutes walk away so thats my excuse out of the way) and the first two times the entire cave was flooded so you were climbing above the water. Have also (when dry )bouldered in the cave on the steep roof section. But yes if you are there go and look but apart from that no no no.
P.S There is a large limestone cave with a little bit of bouldering at the right hand end just the other side of portland near cape bridgewater.