Some of the people on this string are awful.
1) Someone who has been overweight since they were 12 is obviously going to find it extremely difficult to lose weight. (I'm not certain but I believe it's due to the number of fat cells not disappearing even after a successful diet, and overweight people generate and replace more fat cells than thin people) So it's quite hypocritical of people in the comments to suggest that it's easy, when it sounds like no one has been in a similar situation. Also, this means, even if they have a healthy lifestyle, losing weight will be extremely difficult.
2) Overweight people can be healthy, average sized people can be unhealthy. Obviously the OP has said that his diet isn't particularly healthy. But as someone who also has a fairly unhealthy diet, and before the OP stopped rugby he exercised far more than I do, I've remained an average weight for my entire life simply due to lucky genes.
3) The OP has repeatedly said that he is healthy, probably more so than I am, and if he doesn't particularly want to lose weight, then why should he. Yes, being overweight might be unhealthy, even if his lifestyle isn't particularly, but it's his body and he doesn't want to lose weight, then he doesn't have to.
And it should have absolutely no impact on an interview that's supposed to be judging someone's intellectual capability. Saying that it suggests that he can't work hard is rubbish, his results speak for themselves.
Unfortunately, it's obvious just from this forum that weight bias is still very present and that it may be present in your interview. But I really hope that it's not, and hope you get into your first choice uni. x