A bacon joint comes from the fore end of the pig; as opposed to gammon, which comes from the hind. It is likely to have more marbling and is a succulent, well-flavoured choice. It also tends to be slightly cheaper than gammon. Bacon joints are something you may have to visit a butcher for rather than a supermarket. Of course, a gammon joint can be used instead of bacon.
This is a great alternative to a traditional roast during the summer and makes full use of sweet new-season carrots. If you want to cook a smaller joint, I would just reduce the number of carrots but keep the other ingredients the same, and check 30 minutes earlier than suggested.