Isn't the difference here that, while ultimately it is Network Rail's decision to close any route, it is up to individual Train Operating Companies to decide to stop running their trains before that, if they deem it appropriate?
I think that is exactly right.
could it be to do with the position of electrical equipment on the various classes of trains, I.e. if a hst has mechanical/ electrical gear nearer to floor lever, flood waters could affect them worse than a train with electircal/ mechanical parts clearer of ground level?
I think this has a lot to do with it: both
HSTs▸ and Voyagers (as far as I am aware - please correct me if I'm wrong somebody) have axle-mounted traction motors. Clearly, given that in simple terms these are the big electric motors that make the wheels go round, you don't want them dragging through floodwater as that could result in some loud bangs and showers of blue sparks. I don't know if HST/Voyager traction motors have different ground clearances or if
FGW▸ stopping their services first simply reflects an abundance of caution on their part.
Once water has reached a certain depth above rail level, all services will be stopped: if the floodwater is flowing in any sort of current, it can easily wash away ballast or even the earthworks beneath, and this is more or less impossible to detect until the flood has receded, so in some circumstances it wouldn't be possible to be sure that the track was actually still there. For that reason, regardless of the technical capabilities of the rolling stock involved, all traffic will cease.