Yes you can have them altered - I had my Barbour slimmed down for about $20.
One thing to consider when buying one of these is length and how you want to use it.
As a casual jacket, it can be a bit shorter but it will probably be a length that won't cover a suit jacket/sports coat. If you want it to do that, you'll either have to size up or be careful about the model you choose or both.
I bought a longer Barbour off of STP a few years back that is probably around 33" from BOC, so it covers my jackets - sorry, but I can't remember what model it is. I also went up a size to make sure I had enough room underneath to layer and then had the sleeves shortened and the body slimmed. Worked out great