Well I clocked Dover - Cap Griz Nez at 8 minutes, which is 4 minutes each way. A quick calculation should give you your minimum height which will allow you to glide down from any point (bearing in mind that up on top of the White Cliffs at Dover is a couple of hundred feet amsl). And there you go. No busting controlled airspace; no splash.
Always seems neat to be able to see France when overhead Canterbury.