The advantages of under the bump are that they aren't so bulky under tops and to combat exposure or to allow you to wear a shorter top you can always wear a belly band or long vest top under your top top. Under the bump tend to work better for skinny, stretchy style jeans as the stretch in the denim will hold the jeans on your leg as supposed to having to rely on the waist band only.
Spending more than £50 may seem pretty steep for maternity wear, but there's a small army of women who would argue that if you get enough wear out of your jeans, then it's worth spending over £100. I have heard great things, for instance, about designer maternity wear shop, Blossom. They sell jeans by Citizens of Humanity, True Religion and other revered brands with oddly cod-philosophical-sounding names. They adapt the jeans themselves, adding their "Blossom band". A fashionista friend shelled out about £170 on a pair of 7 For all Mankind straight legs from Blossom and says: "I wore them almost every day up until about five months after having the baby, so well worth it." Another friend of mine, who is stickler for quality, got some James Jeans for just over £100 for her first pregnancy and reckons, if you end up having more than one child, it's definitely worth the investment.
Loved the old navy maternity line while I was pregnant! Super affordable and extremely comfortable and flattering. I definitely did not want to go out and spend a ton of money of high end maternity brands if I was only going to wear the clothing for a few months. Definitely would recommend their maternity clothes and will be going back there for more next pregnancy