On all except the bowl gouges it means the outside diameter; this includes spindle gouges.
For Bowl gouges the UK it is measured by the flute, or more accurately from one edge of the flute to the opposite outside diameter. No idea how they came up with that. :laughing: In general a UK bowl gouge is 1/8" larger than a US, PRC, ect.
So a 3/8 UK equals a 1/2 US, a 1/2 UK equals a 5/8 US, etc
Some UK brands are Sorby, Ashle Iles, Hamlet, Crown .. and Packard brand are made by Hamlet and Woodworkers Supply brand are made by Crown...
For bent tools like most spindle roughing gouges, continental gouges, etc I believe it is by the final width not the width of the flat steel.
But you should be careful. As a example the new Hurricane bowl gouges are measured by UK standards even though they a PRC made I believe.
I found out when I ordered a new Sorby and measured the outside diameter of the shaft when I wanted a new one a little smaller. Dang it the new one was the same size as the old. :yes: