As l understand it the 444 and 442s are 23m carriages but the 458s are 20m.
The 458s driving cars are 21m, but the intermediate cars are 20m
The 450s are all about 20.5m
So both types are about 82m long (or 246m for 12 car)
A 10car 442 or 444 train is about 231m (around 15m shorter)
For those that think in old money 246m is 807 feet
I’m happy to be corrected, but I think the Southern Railway electrification schemes of 1930s (including Portsmouth direct and Farnham lines) had platforms extended to a standard length of 820 feet (approx 250m).
I have no idea if a 24.5m vehicle would fit on the South Western, (which would give same length for 10car train), but it is irrelevant to 458 debate (although as East Midlands are using this length, might be relevant to a class 159 replacement (but that is for another thread)