To confirm, the threads on hose spigots are not the same. British fittings are normally a 3/4-inch BSP (British Standard Pipe) thread, or 1/2-inch BSP on some older taps. At first glance American 3/4-inch hose threads look similar to 3/4-inch BSP, but the diameter and thread pitch are slightly different.
American shower heads are normally 1/2-inch IPS threads, which again are not interchangeable with British threads.
As for outside taps being common or otherwise in Britain, I'd say there's no particular regional or other specifically identifiable variations. Some houses have them, some don't, all depending upon whether somebody thought to have one included when it was built and/or whether one has been added at some later date.
By the way, don't be surprised if some people here don't understand the term spigot, as it's not widely used outside trade circles in Britain. To most people it's just "an outside tap."
