Sorry to be a pedant, but C1 is not a structure or rolling stock gauge, although it is often confused, even by some Railway staff as being so.
C1 only indicates that the vehicle is a coaching stock vehicle that fits within the BR standard rolling stock loading gauge.
Coaching stock is 9' 0&3/4" across the door handles and therefore is out of gauge when operating on Goods lines and in sidings when being conveyed by freight train.
If you refer to the ORR website I believe you will find details of the structure and loading gauge, failing that I may have something I can let you use.
Remember that although there is a BR standard loading gauge, each Railway Company had its own variations on gauge, and in sidings for example, clearances are reduced.