A few points regarding Southern announcements mentioned above:
1. Yes, some of the Gatwick Airport announcements are very irritating in length and frequency, but it's mainly the automatic Transec-legacy stuff that drives people mad. Announcements about standing back behind yellow lines are very important and there is a noticeable observed safety risk when they are not made (I should know - in previous lives I have had to stand on the Up Fast platform at Gatwick for 8 hours at a time minus break, performing various duties!). Trains pass at high speeds on Platforms 4-7 at times, and it's well worth getting people to keep themselves and their ever-more-voluminous luggage back. The auto-announcements are OK, but manual ones can have better intonation, so that even if you do not understand English, the meaning is understood.
Unfortunately the same security and escalator announcements are repeated at 0230 on a quiet winter Tuesday morning when staff notably outnumber passengers. This is when the silliness really begins, I have to say! But security announcements are usually no more than every 30mins at quieter times and I've never heard them during normal system operation at frequencies over 4x per hour.
2. Announcing trains on final approach per GTR as above can be a good idea when the calling pattern is very long (as is the case on many GTR/Southern services) and could not be repeated when the train is in the platform due to dwell time limits, or when people are in "turn up and go" mode and do not arrive a long time before the train departs. For longer distance services, I prefer more announcements to be made - as an example, I'd want a train running non-stop between London and York to have its calling points announced very precisely and clearly, because it would be a slight nuisance to require an intermediate stop once onboard...
3. Southern "Please listen to announcements" messages are played when the PIS is faulty and cannot provide audio/visual automated information. There is otherwise the risk of inaccurate scrolling display information, and even occasionally faulty PA announcements. This message is better than nothing, as a prompt as to where to find information, and will be displayed until geographical PIS info is available again, or the driver logs off on appropriate stock. This message will time out on some 171/8 units per current firmware. In a similar vein are the "Stand clear of the doors", "Keep aisles clear" and other pre-recorded announcements, which in accordance with current training should only be used as a supplement to manual announcements if at all possible. These latter pre-recorded options can in turn time out, or be replaced by another. Limited sequencing and volume selection are available for train crew. You can therefore sequence "Stand clear of the doors" and then a reset message "Please listen to announcements" until geographical info is found once again. A caveat, though: I am not 100% sure if all of the above works on 442s/455s - in the former case because I don't have anything to do with their PIS systems, and in the latter case because I've only just got hold of more detailed PIS programming instructions and have yet to have a play with sequencing things.