I don't think they say guaranteed. Its up to 100mb.
I'm guessing you mean down streams on router? Haven't checked router settings for such a long time.
I get what you and others are saying. But as I said for years and most of the time I was getting my full or nearly full speeds. For nearly 2 weeks that's no longer the case that's my main point and find it highly unusual.
Taken from the TOS. Looks like you are in their Minimum Guaranteed Download Speed. 50% of your Speed is their Minimum Speed before you can complain. So it can drop to 50MB and you still won't have a leg to stand on.
Also, not being funny mate but we are 6 pages through helping you try to resolve this. And you just said to @steptoe that they don't provide a Guaranteed Speed. When clearly a simple Google search shows they do.
Least you could do is find out if your speed drop is even in breach of TOS you initially signed up for. Either way your new speed of 60MB is fine according to Virgins terms. If it drops below 50MB is when you can complain...