You have my sympathies - trying to find maternity wear that you like and looks good can be frustrating! Sometimes it really is a case of shopping around, especially as some high street stores (like New Look for example) change their stock fairly frequently. I can only share with you what I've experienced so far, now that I'm 26 weeks.
Next - you're much better off checking out their website rather than their stores. Most of the stores don't stock the maternity range and the ones that do stock very little. You'll get far more choice from the website, to be honest.
Dorothy Perkins - most stores will stock the maternity range but not all of it. Again, it's better to use their website. They do seem to stock more in stores than Next do, however. I've found them useful for every day wear.
Warehouse - their website stocks their maternity range and it looks pretty good! Have been trying to resist the temptation to buy stuff from there, so can't vouch for quality or fit. Not sure whether their stores stock the maternity range.
TopShop - if you can get to one of their bigger stores, you should get a decent selection from their maternity range. I really like their stuff and especially the fact that their maternity stuff is mostly the same designs as their other ranges but in maternity, IYSWIM. Their website is very good and they deliver very quickly.
New Look - it's worth popping in now and again as they do seem to regularly change their stock. I've bought a few things from them now and have no complaints. Reasonably priced too.
H&M - again, not all stores do the maternity range but they're worth checking out for every day stuff. Bit hit and miss at times so again, it's worth popping in every now and again to see what they've got. Again, reasonably priced.
Internet shopping - there are some good maternity sites about and I would recommend checking out; picchu.com, funmum, crave, seraphine and Isabella Oliver. I think that the website for Littlewoods does maternity wear too and at very reasonable prices.
Hope this helps and that you find something you like!