You should be VERY careful thinking that the present figures for intercity tph out of St Pancras and Kings Cross are good indictors for what the UK needs for any new railways.
Compare for example, the first stations over 150 miles from London, which just so happens to be about the minimum distance where high speeds trains really start to have major time savings over the UK’s present maximum speed of 125mph. (Yes, I know they go slightly faster in Kent, but that’s only because they use HS1).
So we’re comparing Sheffield and Doncaster which are almost exactly the same distance from London. Both have a major station roughly half way to London, namely Leicester and Peterborough. Then for the sake of this post, let’s consider the biggest population centres in between, that have rail stations, namely Nottingham and Derby, vs Grantham and Newark.
Now, ECML carries many passengers who are travelling beyond Doncaster. In fact if you look at
https://trainslive.uk/odm/station.php?mode=origin, and take the first 25 destinations (as an approximate comparator), you’ll see that for 2023/24 (latest data on this site), Kings Cross has 8.2m passenger journeys, of which 4.3m or 52% travel beyond Doncaster.
Then if you make a similar comparison for MML from St.Pancras for the first 25 destinations (excluding stations towards Brighton and in Kent to make the comparison fair), there are slightly more passenger journeys at 8.4m, but NONE of these go beyond Sheffield. This says clearly that MML is grossly under-used in comparison with ECML.
You could also compare trips to the four stations on each route as mentioned above. The four stations on MML have 1.5m trips serving a combined town/city population of 1.3m, whereas ECML has 2.0m trips serving a combined population of just 370,000. It's simply down to ECML having had a much faster service for many decades.
Go back 5 years and the figures were even more pronounced, when MML had just 1.0m trips to the same four stations, whereas ECML has dropped from their figure of 5 years ago, which was 2.2m.
There is little doubt that these changes can be largely explained by improvements to the MML service, accompanied by a transfer of passengers away from ECML. Eg. London to Nottingham today by direct train when many used to go via Grantham because it was faster, and similarly many used to go from London to Sheffield via Doncaster, but the faster trains on MML today have drawn passengers to that route.
This post is long enough already, but it demonstrates that the importance of the MML corridor has been grossly understated for decades, and that there is a massive latent demand for greater use of this route. Of course there are more ways of looking at all this, but I hope this provides an indication of how we can misunderstand existing data.