I'll let you into a secret, i love M&S, yes its not the most fashion forward shop but their basics are great and good quality, i love their underwear, they sell pretty(and comfy) shoes and of course the food hall is fab.
Sadly they don't have a huge maternity range, but i do love these two dresses.
I noticed that they also have these fab pj bottoms, they come in navy and black and can be worn over and under the bump and cost from £12.50, as someone in desperate need for pj's these are calling my name. I also think they'd be great for a hospital bag, for the afterbirth bump.
They also sell nighties that would be suitable for giving birth/post birth especialyl the button down one which allows quick and easy access for skin to skin contact.
While you're on the site remember to check out their maternity and nursing bras and babywear.