Not sure how much you can really save but here they are. I would advise others to not over do some of the ideas because you just look really tacky and cheap:

 
Buy the plain white iced cakes from Marks &Spencer or similar and decorate with fresh flowers
 
Buy bridesmaids dresses from ebay or Monsoon/Coast in the sales
 
Rope your friends and family with any skills in to help - decorating rooms, floristry, neat handwriting for invitations and name places, musicians, people with nice cars to drive you, people with big gardens, photographers, …