Full UMS is calculated by looking at the interval between the A grade and B grade, then adding that on top of the A to get the possible A* grade (for AS level there's none given) and then adding once again to get 100% UMS. It goes in this order: 70% is a B, 80% is an A, 90% is an A* and 100% is full UMS
For Edexcel IAL biology and physics, getting full UMS is just 54/80 in most cases, sometimes dropping to 51/80 for physics. For chemistry its around 74/80 and for maths its around 74/75 if you're lucky on the new spec. Old spec is usually 124/125. Hope this answers your question! although I'm a bit late to this