This applies only to the "main" lamps and not the "marker" lamps? Though some vehicles, e.g. the 68, have LED marker-style but main-brightness lamps.
Also I have seen on some of the 313s a "warm" main headlamp and others a "cool" main headlamp; both appear to be normal filament/halogen bulbs however.
Yes, as per my earlier post quoting the latest standard, at night there must be two headlamps illuminated. The way that I read it is that if the train was built to the latest standard, and doesn't have a yellow front, you cannot have a headlamp and a marker lamp illuminated and be able to run during the night. It seems odd given that was the normal mode of operation previously, but that is the way I read it. I can't say for certain with the 68s, but they appear to have a single lamp in each 'cluster' that fulfils all three roles - headlight, tail light and marker (I would guess the
"angel eye" mode) - FWIW, it seems that the 7171, and all of the Aventras in the pipeline (720, 701, WMTrains) are also single lamps at the front compared to the two lamps used on the 345s and 710s.
As for the brightness of the 313's, It seems to be the case that Southern have fitted
HID lamps, whilst the GN units have got more conventional lamps fitted. The slightly triangular appearing thing in the lamp is a giveaway to the HID bulbs.