One member of my last group ride said he used a specialist bike courier which was cheaper than the company (APC) that the ride organisers use. I didn't take a note of the name but it might be this company https://www.bikedelivery.co.uk/.
I'm pretty sure these specialists are acting as agents, offering advice, packaging and a booking service, but won't be doing much, if any, of the actual transportation. I don't
know this, though I do have some logistics experience and for a company to compete with the likes of APC they'd need to be moving tens of thousands of bikes a day!
I've posted a couple of bikes, I use Interparcel as a broker and they usually work out cheaper than going direct. It can sometimes be more economical to split into two parcels, wheels in one, everything else in the other. I don't know who Thorn are using, did someone post on here that bike delivery is currently £100?
The big question is insurance, this can easily cost more than the postage. It's a real money spinner for the couriers, loss and damage is pretty uncommon, though that's no consolation if it's your package that's the one! Many businesses don't bother, if you're sending 100's the occasional loss is cheaper than the insurance.
Then it's just a matter of pack and hope! Well a little thought might help - fragile stuff well protected, I take the rear derailleur off and zip tie it in the middle of the frame. Good box of course, even double line the corners, also one that stands easily upright is slightly more likely to travel that way. Decent handholds make it easier for the handlers and a package going cross country is likely to get reloaded several times. It's possible your box will get stood on, though it isn't likely, I'd hope to pack a bike in a way that it'll withstand being stood on it, though maybe not repeatedly jumped on!
One of those who frequently fly might have some useful advice, If it can stand that baggage treatment, it ought to be fine overland.
Well done of finding a bike, hope it's everything you hope for.