I do and it is far more complex than I outlined - Completely agree in general but Charing Cross is a special case as the platform length issues traditionally limited the number of 12car trains (along with lack of stock!) so BR(Southern Region) took advantage of knowing there would always a fair number of 10 car services (i.e. all metro and some "long distance") but an increase in the number of 12 car services (along with SDO as an enabler) is/will push Metropolitan C Jn beyond being able to cope with 29tph on paper.You do realise that length of the train is, generally, irrelevant in relation to clearing of junctions/platforms,etc. In my experience, having worked on relevant projects, calculated junction margins, platform re-occupation margins and headways are calculated using maximum length trains, so 12 car trains in this case. So therefore are a worse case scenario.
Something might happen in the next few decades at the current rate (along with sorting Dartford to some extent)Worth remembering that the loop services out of Cannon Street are limited to 10 cars in length because of the length of and location of signals on connecting spurs between routes.