If you look at the changes since privatisation, there's not been a *huge* increase in London - Scotland services (a handful of gaps in the bi-hourly London - Preston - Glasgow service filled), despite the big increases on other long distance routes.
This is partly because of the huge share that Air has taken of the London - Scotland market.
We have, however, seen big increases in the following kind of routes
London - Leeds (doubled from one/hour to two/hour)
London - Manchester (trebled from one/hour to three/hour)
London - Birmingham fast services (increased from two/hour to three/hour)
Birmingham - ECML - Scotland (increased from three/day to hourly)
Birmingham - WCML - Scotland (increased from random assortment of trains, many of which ran via Manchester to a faster hourly service through Warrington)
Manchester - WCML - Scotland (increased from a handful a day to bi-hourly, with some additional services)
Maybe, if the London - Scotland market is "ripe" for rail expansion (and given the lack of spare stock/paths for any serious increase) the way forward would be to run more "through" services (e.g. London - Leeds - Edinburgh, more "through" Virgin services running London - Birmingham - Glasgow)?
(or we could just wait for HS2?)