You need to get consent from your midwife to do SW but there is really no reason they would refuse, because its a great healthy eating plan and is, as far as I am aware, the only plan that they will allow you to do during pregnancy. There is no reason why you wouldnt lose weight, although obviously you will regain some as the pregnancy progresses, but you can do SW and stick with it right through the pregnancy and breastfeeding if you choose to.
If you go to a group, the consultants have a booklet available about SW in pregnancy which explains properly how to follow the plan when you are expecting. But do check with your midwife that its ok for you to go ahead.