Even if easyjet did move out of their hangar. I can't see any situation where the hangar would be removed. The old departure lounge is never that busy so does not need expanding. For a start the easy hangar would make great hangar for Signature so they could get rid of their remote hangar with access issues over by cargo.
The main problem at the moment with the easy HQ is that their staff car park is around a mile away and is outside the airport so most staff have to catch a free bus to cover that mile which must be a real pain.
As for TUI they moved their UK HQ from London and not the airport. Britannia ops moved to Germany but no doubt some other Britannia staff found themselves in the TUI HQ just down the road.