I'm trying to use a companion voucher now (1am UK time) - its a PAIN.
I called UK exec club gold line at midnight who advised that I should call back on same number at 1am as its 00:00 GMT when seats are released.
I asked if it was the same number, she said yes. Called back and of course line closed.
Called USA number, got through to Indian call centre - agent said it wasn't possible to add returns onto Avios booked outbounds.
I said it most definitely was and he comes back with, sorry I'm not trained and gives me another US number.
I call that other US number and line is closed.
Call back the original USA number through to Indian call centre again, another agent thinks I should be able to speak to the UK number.

I'm still on hold 20 mins later... will let you know if I get the seats.