I went to virginmedia.com/unlock on the 2nd of August, where it states:
"This form is only for Virgin Media customers who we have sent a letter to and asked them to swap their SIM card"
which you did and you have.
I filled in the form and submitted it. Also on that page it states:
"Once we have received your request we will get back to you via email within 72 hours."
It is now the 13th of August - 264 hours later - and you haven't.