Not for Livestock but Philj W, on RMWeb, says "It was a mobile ticket office placed in service in 1949. Originally attached to a Karrier Bantam tractor unit registered KGY 316. There is an illustration of it brand new in blood and custard livery on page 74 of 'Railway-owned commercial vehicles' by S W Stevens-Stratton. I do believe it was a one off." Was it for shows/exhibitions with the sides opening and what may have been a pull out canopy over the side doors.
I am told (by Merf) that the grey tractor is the livery for a BR road vehicle in "internal use" limited to moving the trailers etc locally. Staines had two separate yards, either side of the mainline, so could have been used for local movement - which would have required public registration.