I agree that the loadings are not great, but I feel they would be greater if they were not turned back at Preston on a regular basis.
Undoubtedly that is true, and there is definitely an interchange penalty, for example Milton Keynes to Liverpool has 40% of the passengers of Milton Keynes to Manchester where both are direct, whereas for Oxford where only Manchester is direct Liverpool has only 14% of the passengers that Manchester does.
That would imply 175,000 one way trips between Blackpool and London with hourly direct service, or an increase of 110,000.
However if you extended the train to Windermere instead, then you would be looking at something like 300,000 one way trips from Windermere (or an increase of 240,000) as you would be doubling the frequency as well as providing direct service. Additionally you would be looking at something like an extra 120,000 passengers a year each for Oxenholme and Penrith. That would give you a total improvement of half a million passengers a year which is a much, much larger boost than you could get out of Blackpool.
I am sure there would be even stronger wins from having additional service north of Weaver Junction to allow you to have faster service to Glasgow and to have hourly service from London to Oxenholme and Penrith and to have direct Glasgow trains calling at Milton Keynes.