I find things that only come part way up my bump just roll down and end up uncomfortable, and I'm told the bump will only go back to about 5-6 months size after giving birth so rolling will still be an issue!
I may become known as granny pants lady but it's true the up to your waist knickers are the best. Get maybe two sizes up from your regular size. You will still have a swollen stomach but it won't be firm like now so they will stay up. You will need the big ones to keep the pads in and they are also good if you end up with an emergency c. Your maternity trousers will help keep them up, over bump being most comfortable after birth IME.
I second Primark! I bought a pack of 5 black ones and didn't leak on any of them. I think they lasted about a good 8 months. Tbh if you do get any blood on knickers if you rinse them in cold water or leave them to soak in cold water then put them through the washing machine they shouldn't stain.