I'd go for at least 130m for most new platforms on secondary lines today especially those for services doing suburban duties around major provincial cities. Sufficient for 5x cars of up to 26m (22x, 80x, 165, 166, Various Alstom things). Also 6x 20m vehicles. On busy non - London corridors I'd consider further safeguarding for at least 8x20m for any new platform construction. 4 car UK FLIRTs, plus their power pods, are the same total length as a traditional 4x20m unit such as GWRs Electrostars. Future Alstom Coradia-based alternatives will likely share a similar architecture and can no doubt be supplied in a similar length. So I'd say 100m should be a minimum anywhere to accommodate 4x20m, 5x20m 4x25m or a single modern FLIRT or Coradia. The next step would be 130m to allow small IETs and Voyagers as well. and various additional combinations of splinters and turbos.