I don't see how that would take less than eight hours end-to-end when the first-stop Warrington Avanti services are 4:30, at which point you'd be struggling to compete with coaches time-wise, let alone other train services (and Paisley Gilmour Street isn't even accessible from Kilmarnock without going via Glasgow or the Ayrshire coast as far as I know).If there was a curve connecting the Snow Hill lines to the WCML, I could see an alternative London - Glasgow service running London Marylebone - Banbury - Moor Street - Snow Hill - Wolverhampton - Stoke - Manchester Piccadilly - Blackburn - Settle - Kirkby Stephen - Appleby - Carlisle - Dumfries - Kilmarnock - Paisley Gilmour Street - Glasgow Central. Although that’s a lot of stops, so it may need to be broken down (London - Manchester, London - Carlisle, London - Glasgow).
Looking at the busiest flows with no current direct services (including and excluding London), and taking ones out where there's no direct service because there's direct services to another station in the same town / city, the only somewhat popular ones now which aren't very short journeys seem to be Brighton – Horsham (Arun Valley might have capacity; I'm guessing the BML and West Coastway don't), Oxford – Bristol (and that's getting at least one direct service from September, I think), Newcastle – Glasgow (as is being discussed in the LNER Glasgow / Stirling withdrawal thread) and ones to / from southeastern branches e.g. Chelmsford – Southend and Henley – Paddington / Reading.
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