Why is Scotsman sometimes deisel piloted ? Because the companies who are being asked to make the move don't always have a steam safety case to operate steam locomotives on the main line
An engine can be towed in steam,As long as it has a current boiler certificate ,and has passed an FTR, but its only for lubrication it can't, or shouldn't provide its own traction,
To be able to operate a steam engine over NR metals, the company operating it has to have an additional safety case in its operational set up with ORR to operate steam traction, as of now, only these have it, Vintage trains, West Coast, LSL, and DBC, Eastern rail are seeking to add steam to its porfolo,
Any other organisation, That's registered with NR can move a steam loco on NR, It can be in steam, but should not be used to provide traction, if you don't have the required permission to operate a steam loco on NR.