"Before you depart" the airport!

But seriously, unlike in the US, it's not easy to just walk out of the departures are back to landside. I'm pretty sure there's no border control upon exiting the UK (like Schengen), but there's no public "path" back to landside because there's generally no need. However, I believe an airport staff member can escort you (there's probably just a door somewhere, but it's probably fire-alarmed for emergencies, because obviously you DO have to leave a way for people to exit the building in adverse circumstances).

But I think the "real" issues are (1) getting through security back into T3 departures without proof of an onward flight, and (2) whether the lounge policy actually allows DL Plat/DM to use the Clubhouse without an onward boarding pass.