If you are in a UK size 8, you should be just fine. I think that is a US size 4, which is plenty tiny. Concentrate on your exercise so you stay strong and toned, your weight doesn't sound like it is a problem.
BMI is a bit different measurement than whether you're over or under weight. It's an attempt to measure how much of whatever you weigh is fat and how much is lean tissue or muscle. If you look fine you probably don't have a weight problem, but you might have an eating problem even if it's just small. Health is a different measurement than weight. There is much evidence that overweight people don't actually die that much younger than others, but they do suffer from more ailments along the way. There is some evidence that leaner people are healthier than those who have more body fat even if the leaner person might weigh more. Muscle isn't light. There is also contrary evidence that is a bit in question now that came from rodent studies that showed that eating too little extended life -- I think those studies have been challenged now and don't now if they're still good studies. Basically, health is a lot more complicated than weight, though the two do have some overlap.