The OFCOM procedure is that your new provider deals with the ENTIRE porting process. This INCLUDES taking over the cancellation of your landline account.
As you have given Virgin notice to quit, your landline is now classed as "pending disconnection" & any porting request from another provider will fail. If you want to keep your Virgin number you must contact Virgin again immediately & re-activate the account for your landline or you will lose your number.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.
I called virgin media and I was told by an advisor that I can take my number with me before disconnection happens and it does not matter if I have given my notice to cancel. So I am confused now as to who is correct.