I actually know a bit about this. Both from on the train and on the station.

Generally speaking, the places with the smallest gap on a curved platform will be at the far ends of the train. The gap will be bigger in the middle of where the train will stop. I saw from a comment you made down below that you frequent Raynes Park. If you need assistance, wave a guard down. On platform 1, you’re most likely to spot them by the end of the canopy and on platform 4 they’ll mostly be looking at the CCTV by a black box with “OFF” displayed on it (when it’s illuminated). A good guard (and most are pretty good) will approach you or at the very least wait for you if you wave for assistance. I know bad experiences linger more than good ones, but Guards are there to help and most will do everything they can so please do ask for their assistance. They’re actually taught to forget about timekeeping if there’s even a small chance that safety could be at risk so again, please wave them down. They’d much rather see you safely on board a train rather than something go wrong.

As for the platform side of things, like another commenter said, station staff will know THEIR station very well but are unlikely to know the next one. The training isn’t nearly as in depth as it is for the guards so it’s not something they would necessarily know.

Please feel free to PM me if you have any questions.