There isn't a right in consumer law to get a PAC code, although it is a requirement of the regulator Ofcom that companies provide a PAC code for active numbers. If VM breached that requirement they'd be fined.....but before you get your hopes up, the company has been alerting customers to the end of the PAYG service for months now, and under similar regulatory conditions telcos can't "sit on" unused numbers, these have to be recycled. Since VM have contacted customers, sent reminder texts, and notified Ofcom that they are withdrawing their PAYG offer I don't think there's any chance that they would see VM's action as wrong, and your rights of redress are probably non-existent.
I appreciate it's not any help now, but customers needed to get the PAC code when the number worked, or in the circa ten days grace between the account being barred from outgoing calls and the account being fully suspended (seems to be the policy). If you're being told that you can't get a PAC code, then likely the account has been suspended and number has probably gone into the recycling pool. If that's the case then it's not just that VM won't issue a PAC code, even if they did it wouldn't work, because the systems of the receiving company rely on the number being active.
I'm guessing there's probably no way VM can/will make your account active again with the number, so my recommendation is to accept that it's gone, get a new SIM or contract as you see fit, and start letting people what the new number is.
