I really like some of the clothes on Dorothy perkins at the moment, and river island and also asos - granted many of the dresses are uber short but the midi length is also quite fashionable at the moment so there are some of those about. If you search "midi dress" and "midi skirt" on asos as well you will find some ideas too.
Toast has some interesting dresses. A little more expensive but still under £300 :)
Seasalt new spring collection has a couple of very pretty dresses. Admittedly more cotton and summery but really nice and reasonably priced.
I would also look at larger department stores online like Debenhams and House of Fraser. May be some special offers to browse through.