I am afraid that small increases in speed dont help journey times much . what really helps is eliminating permanent speed restrictions. the more severe the restriction that is eliminated, the better.As often noted, 140 vs 125 is 3 seconds a mile, you need a lot of it to make a dent. Its not just track you need to upgrade either.
So where are the places on the 'inter-city' network that would benefit from a few JCB's doing their stuff?
This curve-radius would allow 100 round the corner, and switches for those speeds are well-established railway technology these days.