I'm faced with a similar issue.
I also upgraded from Ultimate Oomph to Ultimate Volt, with the only difference between the packages being the unlimited sim is now O2 instead of VM and broadband speeds should be boosted from 600Mb to Gig1. When I upgraded I was also told the same speil as OP that the Gig1 is available in my area but the Super Hub 4 was out of stock and I won't be able to get the Gig1 straight away, however, I was told that once they are back in stock I will automatically be sent the Hub 4 and be upgraded to the Gig1 at no extra cost. The next day I get my contract emailed to me and it tells me that I'm still on the M600 with no mention of Gig1 anywhere. Under broadband on MyVirginMedia app, it tells me that one of my Volt benefits is "Your broadband speed has been boosted to the next available level (if you aren't already on the fastest speeds you can get in your area)" but this is not true as I've been on the same speed for almost 18 months.
Since then I have rung Virgin Customer Services multiple times, and spoken to multiple departments (mainly retentions who did the upgrade) to get them to clarify the exact details of my contract. Almost every advisor I have spoken to has been contradictory. Some are saying that it isn't on my package however they can process the upgrade right now and get the Hub 4 posted out to me but there'll be an additional charge, some are saying that I am eligible for the free automatic upgrade once the Hub 4 is back in stock and that I just have to be patient, and others telling me that they can process the free upgrade but their system won't allow them to build the package and process it. I was expecting a callback yesterday (that never came) from one of the advisors, Susanna who was chasing their Team Leader for updates about my problem.
After not receiving the call, I contacted VM once again. This time the advisor informed me that because it's been 15 days since I upgraded I can no longer back out and cancel my contract, however, he advised me that I should complain and get my upgrade call listened to by management and anything that was promised over the phone I'm entitled to receive. He also advised me not to process the complaint today as it would "rock the boat", instead I should wait for Susanna to call me back with an update.
Needless to say, I'm not happy about the whole ordeal.