EagleJCS said it all re the BSA in the US. The only proviso is that the circumstances where uniform is actually required are quite limited, and on other occasions a scout shirt worn with non-uniform trousers, or with a kilt, is OK.

The Scout Association, which is the UK body, allows kilts as uniform, but with complicated rules that depend upon the scout or leader's ethnicity and the location of the unit within the UK. I think Irish ethnicity allows me to wear a saffron kilt only. Scottish ethnicity lets you wear your clan tartan or the scout tartan outside Scotland, but AFAIK you can wear any tartan in a Scottish unit whether you are of Scots descent or not. And I'm pretty sure the rules are more complex than I can recall. I think I posted them in another thread at one time.