Some women seem to lose weight rapidly whilst breastfeeding, others don't seem to finish losing pregnancy weight untill they wean. Its certainly not putting me off breastfeeding my first baby.
I completely understand why there is so much variation. To begin with, we are all starting pregnancy at different weights and gain different amounts during pregnancy. Some people will be more affected by changes in appetite than others and I'm sure that we also react differently to hormone changes. We eat different amounts whilst we are nursing, and some babies will require us to be more sedentary for longer feeding than others.
It doesn't surprise me that some women lose a bit of their weight when they wean, because there will be changes in their breasts, they might go out and about more (no longer taking long nursing breaks on the sofa), they may have a ore mobile baby to chase after and they might cut out those snacks they ate whilst they nursed or straight after. Not to mention any hormone changes or the effect in a change of routine if this coincides with a return to work. But it doesn't mean that most of the weight won't go till then.
If you've only put on 2 stone at this stage in pregnancy, you'll probably find you do lose most of it in the first week or two after birth. I really wouldn't give up breastfeeding just because you may hang onto half a stone or so... there are so many benefits to you and baby, and you won't be nursing forever. You might find that this time the weight goes down anyway.
I totally understand wanting to lose weight after the baby is born. I was losing weight pre-pregnancy and still have another 6 stone to lose. Because of starting out overweight and being careful, I haven't really put on weight during pregnancy (35wks) though, so I can't say its baby weight! I'm going to do my best to keep to a healthy diet, and I'm likely to be more active once the baby is here than I was before (baby wearing, plenty of walks, etc). I think I'll lose weight but I might not get to my goal whilst I'm nursing. I won't resent the baby for that.
Anyway, you don't have to make a decision now to not start breastfeeding. You can wait and see how you feel and how your body reacts to nursing and take it month by month. You won't be feeding forever, there is plenty of time to lose the weight afterwards - and not everyone does hold onto much weight whilst feeding, and some lose it!
Enjoy your baby whatever you decide.