I don't suppose that trying to save on traction power costs will be as big a motivator in the future as saving on fuel is today, and the faster-accelerating trains of the future will also be able to spend so much more time at maximum speeds of 140 km/h. A TRAXX platform locomotive with maximum power 5600kW may haul a 5-car train, of weight ~350 tonnes including the loco, and with a starting tractive effort of ~315kN. Today's MP40PHs may, with output 3000kW, haul 12-car trains within striking distance of 800 tonnes and with about 370kN starting tractive effort. Some quick calculations can produce ballpark figures for a stopping penalty - i.e. how much extra time it takes to reach a distant point after coming to a stop and accelerating again, ignoring the time spent dwelling while stopped. For current trains, it comes out to well in excess of 200 seconds for a MP40PH and maybe a dozen seconds less for a MP54AC. The short electric train may do it in 90 seconds. Modern EMUs could halve that again, but I understand they aren't planned in the near future for pragmatic reasons.
I was really shocked at the old Uxbridge sub speed limits though. Do we know if they've been raised following the double-tracking project?