As a child in the 1970s or maybe early 1980s I read a science fiction novel about a couple of airmen (probably RAF) in World War II who crash their plane on, or parachute to safety and land on, a deserted island in (I think) the Atlantic. They find a crevice in the rocks of the island and follow it down (or is it a sandy cone they are sucked into?) eventually finding an entire albeit small, civilisation deep underground at the foot of an enormous cliff. They use their parachutes to float down to the hidden country and somehow they also manage to get out later. Does anybody know what this might be? I have the feeling that the book itself might have been published earlier than the 1970s. Edit: I think the first name of the author may have been Bruce (?) and was probably British.
Any chance this would be "The Perilous Journey: Into a Strange Lost World" by Bruce Carter (pseudonym of Richard Hough)?
It's a young adult novel that was published 1958, so the timing seems about right, and the author was British. The plot details as summarized on Wikipedia also match (in broad outline -- there's not much detail there).
I haven't read it myself, but it seems a close enough match to bring it to your attention.