I've been searching and searching and cannot remember the name of the fic or where to find it but I remember Buffy went down to South America, maybe Brazil, to track down Spike and he was nicknamed el Diablo Rubio. He agrees to help her and they go to England and there is a watcher/warlock named Collin. Also a leprechaun friend of Spike's. Then also something with Stonehenge and spike getting lightning electrocuted.

I know it doesn't fit the bill completely, but I thought it was worth mentioning because of the Watcher named Colin and the travel to England then Scotland. Your description made me think of Charms of The Clarion by Eurydice.
On EF, ff.net, AO3 and SA.

You Just Fall by Sabershadowkat has Buffy run into Spike in Mexico and he's called El Diablo Rubio. It doesn't have a wathcer called Colin though.

https://www.sabershadowkat.com/buffy/youjustfall1.html