Could the guard close/lock the doors at their own discression though even when the signal is not showing a proceed aspect? In theory they could ask the driver to draw forward to the signal.
Yes they can as long as they follow the rulebook.
6.8 Moving a train before station work is complete
If it is necessary to move a train before station work is complete, you must make sure all the doors are closed before instructing the driver to make any movement.
Before any movement is made towards a signal that is at danger, one of you must get the signallers permission. You must tell the signaller when the movement has been completed.
6.9 Moving a train on a platform line where permissive platform working
is authorised
Permissive platform working allows more than one train at a time to be on the same platform line.
You must not make any movement, other than for coupling or uncoupling, once the train has come to a stand unless:
a signal is cleared for the movement, or
the movement is authorised by the signaller.
If the movement is made on the authority of the signaller you must tell the signaller when the movement has been completed.
I disagree. No doors close until the road is set. That way if late comers turn up they can still board and not ask why they can't board 'they're train' despite it still being in the platform and not moving. Okay, the train is late, but it's still going where they want to go and it will save the argument if they're just allowed to board. Ergo, no CD until the aspect is set to proceed. I've never had a problem with it, because most people turn up on time for the train and therefore are already aboard. I've seen the reverse problem - where someone has given CD on a red and then been taken off dispatch...
I quite agree which is why I take the same approach, if the train isn't going for a while I also let passengers on the train to go onto the platform and stretch their legs.