A reality check before this gets too surreal. We need haste not speed.
Biggest savings come from raising the lowest speed limits.
So if you really are going to send Cornwall services via Badminton and the rhubarb curve (I can just imagine the press coverage!) then raising half a mile from 10 mph to 15 mph will save a minute - as much time as raising 19.5 miles from 125 to 140.
I made a similar point after the Tornado trip a year ago - lifting the 5 mph steam clearance slack through Bath (to the usual 40) would
have saved more time than allowing 90 mph or more down Dauntsey, if rather less fun.
Back in the real world, yes the overly restrictive limits out of PAD need sorting, ditto the 15 mph into plats 13/15 at BRI. Far better (and easier?) to do that than contemplate 140 mph running.