It's been my perception and understanding that SRPS have had a close and co-operative working relationship with WCRC, with regards to rolling stock.
WCRC providing motive power for empty stock moves...
And SRPS hiring carriages to WCRC when their own fleet has been fully deployed, ie at the height of the summer tour season, plus Jacobite, Scarborough Spa, Welsh Mountaineer, etc.
I have certainly seen SRPS carriages, 3 or 4 for instance, formed within a rake of otherwise WCRC stock, on excursions passing through Preston, in recent years. (The SRPS vehicles appearing somewhat 'weathered' in comparison, and with the wording 'SRPS Railtours' in gold on the maroon.
Whether the same working relationship continues, I cannot say..