Saying that she is worried about what sort of dress your gonna put her in suggests that your wedding is a long way off yet???? It's very common for family and friends to have an air of indifference until it gets nearer the time. Although we want to talk about & plan our weddings for years everyone else is not so keen! My Mum didn't really get excited with me until about a month before but in the end she was fab
Maybe if she mentions the dress thing again, you should enquire as to whether she's happy about being your BM. Maybe tell her that you thought she'd be really pleased to be one and that the last thing you wanted to do was make her feel uncomfortable and it's really important to you that she enjoys the day too. Then if she's not 100% into it she may take that opportunity to back out.
Regarding the hen do, personally I feel that if you've decided to have a weekend in France then you can't really moan if everyone can't make it however much you want them there. If your friend is still keen to be your BM then maybe you could suggest a UK evening out (meal/pub/bar e.t.c.) that she could be in charge of organising?
Let us know how you get on x