Does this matter? There are up and down slow lines and passing loops to accommodate slower trains such as freight and preservation excursions on both the WCML and ECML. Also once you're into Scotland there's very little in the way of 125mph operations. The joy of a steam or historic diesel excursion isn't the speed, it's the being there and enjoying the nostalgia of olde world locomotion and carriages.
Also if you're paying 150-300 GBP for these outings you want the experience to last, not just four hours up the ECML at 125mph and then you're done. I kinda feel people miss the point of this as a thing.
The problem with the speed isn't for the passengers on-board—it's for the rest of the railway.
It's one thing to deal with the operational complexity of relatively slow services when they're freight—which provide a clear commercial and economic benefit—but it's another when it's essentially a luxury service (and I'm well aware plenty of the services aren't really luxurious: but they're certainly too expensive for many, which makes them a luxury).
Their acceleration is relatively poor even compared with many freight services, and when running up the northern ends of the WCML/ECML (both of which are two track railways) the challenges of timetabling _any_ slower services during the day are real.
Well these tour operators do pay for track access and pathing just like other TOC's and the rest of the railway will just need to get on with it.
Also when it comes to rail tours such as this there's at most two tours a day operating and they're very seasonal, and maybe running four or five times a year each. So a couple of these trains a day across the whole network isn't exactly impacting the network that much.
Also if you're paying 150-300 GBP for these outings you want the experience to last, not just four hours up the ECML at 125mph and then you're done. I kinda feel people miss the point of this as a thing.
ps: extreme train enthusiast speaking here :)