Mamas and Papas have up to 70% off a range of kids clothes and also maternity and nursing wear. Looking at it I wish that I was having my baby now, rather than two years ago when all the maternity styles were horizontally striped lines! The nursing outfits (that can also double as maternity tops, so you get good life out of them) start from £7, and the most expensive is £13. In the maternity category as a whole there's nothing over £27, which for a little black number with a big belly is really not a bad price at all.
Some items have click and collect available, you just need to pop in your local store and the system will show you what you can collect, some as soon as one hour from ordering (I guess based on time of day!) If you need to have it delivered, your delivery is free if you spend more than £50, and £4.95 otherwise.
If you aren't sure what you need to see you through the bump & beyond, have a look at Heidi's guide to your maternity wardrobe for around £100.