Get one which starts under your bust and goes down to your knees - less chance of any rollage that way. Ones that start at the waist cut you in half, ones that don't go past mid thigh will roll up your leg (and Visible Spanx line is worse than VPL)
And get it for the size you are - don't think a smaller one will suck you in more - it will just have more risk of starting to roll. So if you are a 14 get a 14/16 not 12/14 - will be more comfortable
Primark's are as good as any I find