Out of curiosity, I have a related question...
Are guards trained to watch out of the window as the train departs to check that the train has cleared the platform without any issues? I've noticed that sometimes when I board a train where the guard stands they will do this - they won't lock the door panel until the last car is clear of the platform.
If this is the case, what exactly are they watching out for and what could they possibly do about it once the train is already accelerating?
However, it's not consistent. I'd estimate that the majority of guards don't do this and will talk to you, sell you a ticket, etc. while the train is leaving the platform.
Yes, this varies by TOC. A lot of SWT guards as you say, will be making announcements or helping passengers as soon as they give the right away to the driver. The other thing to note is that guards on SWT can despatch the train from any part of the train (on the desiros and the 158). So on a ten coach train, guards can despatch the train from the very front if they wish and so even if they watch out of the window, they are certainly not going to be able to watch the entire train leave the platform.
Compare this to GWR. They are much stricter about this and you will always find their Train Managers despatch the train from the rear or towards the rear even though CDL panels can be found throughout the train. With the HSTs, they will have their head outside the window and one hand on the CDL panel ready to give the signal if necessary. On the adelantes, they will be square on with the door looking outside. Note that there will be 0 distractions so even a passenger has an enquiry, the Train Manager will politely tell the passenger to wait. Announcements will not be made until after the despatch procedure has been completed (i.e. train has left the platform).
The platform staff will do the same. So as the train passes them, they will say bye to the Train Manager (not that this is procedure

) and then get perpendicular to the train/platform and keep watching until the entirety of the train has left the platform.
So the despatch procedure varies by TOC. I was not criticising SWT guards by the way and not suggesting they are not following procedure. But I guess SWT management or the way they are trained up is so that looking out of the window is not a fundamental part of the despatch procedure.