Ring them and ask what days they slaughter and how much they charge etc, they are a business just like any other and will probably be most helpful. With a single animal, you will probably need to take it in, and it will probably be better for the animal also.
With regard to if it's worth it . . . . well, you get a lamb in your freezer that you have reared on, at a decent cost, and get to try a 'minority' meat, and see what you think.
I'd do it.
But then again, you may get a bit attached to the one.