It’s the car battery that’s flat, and this being a 2006 A3, it has a visible conventional lock on the drivers door, but nothing on the passenger side. I’m still of the mind the passenger door is bust, any door should be openable from the inside,no matter the state of the central locking.
If the car has deadlocking, it may not be possible to open the passenger door without the central locking working. That's the whole point of deadlocking - you can't smash a window and reach through to pull the handle.
In normal operation, once the car is unlocked, the deadlocks are off. Even if lock it from inside (or it autolocks on drive off) by pressing the lock button, it doesn't deadlock and you can get out by pulling a handle in an emergency. You can only set the deadlocks from the outside, either by the remote or some incantation of double or long locking using a key.
Interestingly (or not) the hidden mechanical lock on at least some JLR products is in the passenger side door, not the driver's.