London-Leeds-Glasgow is not going to happen, but Leeds-Edinburgh-Glasgow does involve a dog-leg via Carstairs. If you could raise the S&C to 90mph ruling linespeed and guarantee a 6-car 185 every 2 hours (don't think it would need more than that) then it could offer a useful link. Problem is of course the cost of the interventions required on the S&C will be out of proportion to the benefits.
I don't know how much of a time saving you'd get from a Leeds - Carlisle - Glasgow service versus the current Leeds - Edinburgh - Glasgow service that XC run every couple of hours when there's not a major global pandemic taking place, but it'd be significantly easier to link Leeds and Glasgow with more trains by just extending the current Leeds - Edinburgh terminators - no need for more paths up Beattock or anything complicated
(then again, I'm not that convinced that Leeds to Glasgow is a huge market - it's a bit like Derby to Manchester - it's something that gets mentioned on here because suggesting that there's a big demand between the two places is a convenient justification for trying to get long distance trains running along a scenic route - the S&C in the case of Leeds and Glasgow, the Bakewell line in the case of Derby and Manchester)
Being one of the very few people to have travelled to Center Parcs in this country by train, I can advise that even if there was a station at the front door of the place, the number of people who would use the train to get there would be minimal. You just have to look at what is contained within the average Center Parcs customer’s car on arrival to work out why.
I appreciate I should have been clearer - I was meaning more that it would be the kind of hook I'd look for if I was wanting to promote this project - regardless of whether anyone was going to use the train for a family holiday there (in the way that developers like to use the hook of a freight siding as a way to justify their distribution centre (even though they know that there'll be thousands of lorries using the place whilst the siding slowly goes rusty)
The S&C line has never made money and never will. Nevertheless the losses incurred by the line are a tiny proportion of the overall subsidy needed for the current extent of passenger railways in the country
The same is true of every line though - you could justify propping up any lightly used line on the grounds that it was only a tiny proportion of the overall subsidy - even the Conwy Valley line (millions of pounds in repairing it each year, but significantly fewer passengers)
It is not redundant in times of disruption on the WCML and ECML(for diversions)
When as the last ECML diversion up the S&C?
In fact, I'm struggling to think of the last WCML diversion up the S&C.
The S&C is the kind of line that,
if it had been closed in the 1980s, we'd have regular threads on here blaming Beeching for it(!), we'd have lots of people claiming that, if only we re-opened it, the S&C would be used for ECML and WCML diversions most weekends, there'd be long distance services that way (for the all important Nottingham to Glasgow market!), there'd be bespoke tourist trains too, maybe some London trains as well, if only we could re-open it. The reality of the S&C is pretty underwhelming though - a Sprinter every couple of hours and nothing north of Carlisle - which is why I'm always suspicious of proposals for just how apparently amazing a proposed re-opening would be.