There are lots of rules based around historic speeds. I suspect the rule is only 125mph because that’s simply the speed in which all but HS1 goes. Had the HST been developed to travel at 135mph and the infrastructure allowed it, I suspect that ruling would have been 135mph.
IIRC the rules regarding yellow lines and warning notice requirements were first brought in for speeds of passenger trains above 100 mph, and freights above 60 mph (more turbulence), and when the rail group standard was still publically available a couple of years ago this hadn’t changed. But as we know yellow lines are applied in numerous places where they’re not really required per the rules.
I don’t think there’s any difference in allowed speed for new build platforms, however the width of a new platform would presumably have to allow for the intended speed with no dispensations on the clear area being allowed.
As you suggest the 125 mph upper limit is probably fairly arbitrary, I expect when the HST arrived on the scene they didn’t expect anything much faster…
Edit to add:
Something I can’t remember, did the introduction of container trains result in the simultaneous introduction of yellow lines (for excess turbulence), or did they happen retrospectively after analysis of safety incidents?