Actually, in the UK, mutton tends mainly to be eaten by members of Asian, Middle Eastern and Caribbean communities. It's not widely sold in supermarkets all over the country, unlike, say, lamb. And where it is sold in a supermarket it tends to be one product line where there might be several for more common types of meat like beef, pork and lamb.
Yes. Mutton is the meat of the adult sheep, lamb is the meat of a juvenile sheep. The flavor is different, but in a volume-ish way. If you think of the flavor of lamb as being a 4, the strength of the mutton flavor is like a 7-8. It's the same flavor, there's just way more of it.
Mutton is really goat. Lamb is just lamb meat. IMO, lamb is sometimes harder to cook because of the smell - you need the right spices to get rid of the smell. Easy to get mutton in most urban areas in the west - just look for a Halal meat shop.
