I posted this on another thread but it may help this query.
My wife's is phone is PAYG, bought more than 3 years ago. She received her new SIM yesterday and when it was inserted, it worked without using the personal unlocking key. She noted, though, on the red card containing the SIM and under the SIM are 2 items: your default security PIN and your personal unlocking key. The latter is very faint and may require a magnifying glass to read but it is there.
I hope this may help. I am not an expert, just another Virgin customer trying to deal with the SIM change with no information or, as yet, response from Virgin.