I've bought a few cheapish Maxi dresses from the likes of Peacocks, Next, and George, they're not even maternity dresses but because of the way a maxi-dress hugs the bust and then hangs there's loads of room in the skirt for my bump. They're lovely and cool because they're cotton and there are some lovely colours available.
I have really struggeled with maternity clothes and like to wear dresses. I have bought non-maternity maxi dresses. Am currently 36 weeks and there is still plenty of room for the bump. Other suggestions are Seraphine (they have an online store but also a shop within minutes of High Street Kensington station in London). I have bought from there and the quality is good. ASOS also do a selection of maternity clothes. I can't comment on their quality having never bougth there but htey do have a good choice. Good luck.
I bought a few dresses in the French Connection sale - 1 maternity (on Asos) and 2 non-maternity just a bigger size in the shop - one is really flattering and I got lots of compliments on it, it's strapless with silk bottom part and can be dressed up and down, quality is very good and with sale prices at 50% god value for money too