I can't say I know specifically of a uni that does a PhD in sports supplements but I suggest you contact a couple of depts. and see if they are interested. I was going to do a sports science PhD and I know that Exeter and Brunel are two of the top unis for sports science. I just emailed the dept. at Exeter and the head put me onto a great Dr. who was researching in my particular field, very helpful. I am going to be at Brunel now, however and I know they have a good dept. too. I am not going that route any more I am doing a neuroscience PhD using exercise as a factor.
Not sure if this helps but there you go. I didn't mention Loughborough!