Very quietly, a handful of Train Operating Companies running main services have started to employ more "heritage" units on regular "main network" services. The following have come to my attention:
Cardiff Bay services (A single car class 121 "Bubblecar")
Aylesbury to Princes Risborough (A similar unit)
Brockenhurst to Lymington Town and Lymington Pier (A heritage electric)
Ryde to Shanklin (about 6 heritage London underground trains)
It is said that the cost of purchasing outright, and restoring, the class 121 unit for the Aylesbury line was less that the cost of leasing it for a single year. Wikipedia also shows a considerable number of other similar heritage units in service with Network Rail and from time to time, that fleet turns over. Indeed,
wikipedia shows one sitting out of use at the current time.
You're looking at - what - around 170k to 200k per annum to hire a more modern unit (that's assuming you can get one), and we are told that First - at a time when they understood the line's growth to be less that 1% per annum and had no first hand knowledge of operating the train service - bid 300k per annum to the government to run a respectable service under SLC2.
It looks like there might be mileage in a suggestion to "do and Aylesbury".I note ... the Aylesbury line runs from a major town across to a junction on another line (compare Swindon to Westbury) ... and that the unit has been fitted with door mechanisms to mean that it meets the standards required from November 2005 that ruled out unmodified heritage units on such services after that date.
I also note that two of the four heritage services that I've mentioned above are run by the SWT franchise of Stagecoach - giving that company experience of such an operation. And they have a depot with diesel maintainance facilities sitting at Salisbury.
Two "bubblecars", passing each other between Trowbridge and Westbury, providing a 90 minute service between Swindon at Salisbury. From Salisbury at 06:00, 07:30, 09:00, 10:30, 12:00, 13:30, 15:00, 16:30, 18:00 and 19:30. Returning from Swindon at the same times. Does this look attractive? And with FGW continuing to run their extras from Swindon at 06:18 and 18:45, and from Westbury at 07:00 and 19:35. you have an appropriate daytime service with peak strengthening up to Swindon. I do hope that crossticketing would be available!