I suggest 3 ton class rather than 15 cwt. I base this on the tyres appearing to be more likely 20" than 16" (matching the truck ahead) and the bridge class 8 being typical of 3 ton rather than the 5 which would be typical of a C15A.
http://www.mapleleafup.net/forums/showthread.php?t=6062
The actual bridge class of a 3 ton vehicle could vary depending if it was mainly an empty box (e.g. a dental office) or crammed with metal bits (signals construction).