Paul Hutchinson wrote:

The East Coast Main Line is currently way below the capacity achieved on
the Southern Main Lines every day.

Of that there is little doubt, in general. But then they are very
different railways, in general. The one which possibly comes closest to
the ECML in character is the SWML, and that has many similar capacity
issues to the ECML, with much the same causes.

(I think it's a great pity that the DN&SR, which was perceived as a
strategically important railway and upgraded in WW2, was allowed to
disappear, but that's an entirely different topic).