If you're considering breast feeding it's worth looking at pregnancy clothes (maternity or just sized up) that might also be bf friendly, so nursing tops/dresses or layered vests, etc. I didn't think about this last time and so shortly after DS was born I realised I had nothing I could feed in (normal clothes too small and most of my mat stuff was not suitable). If something can do you pre and post pregnancy then it makes it much better value. I bloody hate clothes shopping and especially mat clothes shopping!
In addition to the retailers others have mentioned, I got a few maternity and breastfeeding clothes from Mothercare last time. The selection is not massive, but they regularly have new stock online and useful reviews! The Blooming Marvellous range had some good dresses and nightwear before. Look out for sales as they seem to happen fairly often.
Oh, and I got a couple of smart dresses for weddings (mat and bf friendly!) from yummymummymaternity.co.uk worst website name ever. I also got an absolutely brilliant winter coat from there too, and in the sale just before Xmas. If you sign up to their newsletter you sometimes get sent discount codes and they have regular sales and special offers. Good delivery and returns policies too.