Hi antani
You won't receive new customer pricing but you should be able to negotiate a new package deal with VM. You can always chat with O2 and choose a different sim tarif, but I suggest using uswitch for your mobile sim since you are out of contract.
When I was out of contract I checked the sim pricing on the O2 website, and then compared that pricing on uswitch <here>
With uswitch.com, you get the option to login to your O2 account, then if you select that you don't want a new SIM card sending out it lets you keep your current O2 number and starts a new 12 month contract at that new price which nulls out (cancels) the price you were paying O2.
I changed my O2 sim via uswitch and am now paying O2 £10 a month for 50GB (boosted to 100GB) over 12 months and kept my Volt benefits on the VM side too. (This would be £24.00 per month on the O2 website over 12 months
uswitch are currently showing 6GB data for £6.99 a month, and 30GB data for £7.99 a month both on 12 month contracts.
On the O2 site the 6GB data is £20.00 a month, and 30GB data is £23.99 a month both, on 12 month contracts.