I didn't buy the special bra and undies, because seems expensive ... for Bra, I got some new normal bra with no under-wire (feel more comfortable) with larger cup size, then add extension at the back... the bra can be worn after delivery and much cheaper.
For Undies, I also think no need to buy special, you can go Marks and Spenser buy undies (pack of 5) full brief at a larger size than your normal, that would cover the belly well and the cotton with lyric fabric is quite comfortable. you will need quite a number of undies for keeping clean and hygiene during pregnant, therefore, better buy cheaper price but more number for change. Esp. if you need to do housework and laundry yourself.