It's P1 that's used, for Overground and Southern stoppers, and P2 where the doors aren't opened..
Almost. It's correct that P2 is an unused platform face on the same track as P1, in years gone by it was often used by people changing from a slow to fast line train, or just leaving the train; no doubt it was annoying for staff.
In the years after slam door there were still a handful of peak hour trains that were fast to London from P1 not P3. I spent many years knowing the (I think) 0848 which was a Caterham and Tattenham Corner joiner, due on P3, would quite likely be late and the 0851 from Sutton, timetabled for P3 too, would arrive on P1.
These days, as far as the passenger is concerned, there's no such thing as a fast London platform. Since the Thameslink revolution the platform depends on where the train came from, not where it's going. Fast London trains from the West Croydon line arrive on the slow line and cross over to fast.
No doubt this makes sense when it comes to timetabling, but it's awful to the passenger