If you really don't care too much where your cabin is, and don't use the MDR, then a saver fare could suit you. However, check what you might get in the way of obc/parking etc if you were to book a select fare. Sometimes the difference can be quite small, and parking at Southampton can be quite expensive.
If you go down the saver route, basically you are tied into that cruise, so cannot move your deposit to a different cruise.