VM staff are told to paste the scripted response from management
You'll just have to suck up the price increase and pay if you are in contract and don't want to use workaround that I will describe.
This is a known workaround but would restart an O2 contract but it's a way of getting a cheaper O2 tariff if you are on say £10 or more.
Visit uSwitch.com, select the cheapest O2 tariff, or one that suits you, then go through checkout. During checkout elect to sign into O2 as it's an O2 checkout. Select NO SIM required, and your contract is renewed for 12 months under a cheaper tariff or same tariff with more data allowance, depending what you picked. Volt benefits will reapply double data within 14 days along with travel allowances for roaming. I believe the O2 contract and VM contract needs to be in same name and address still.
You will get threats during checkout of early termination charges, but this disappears later in checkout as O2 actively market you can change tariffs during minimum term anyway for years.
Many customers including myself have changed their tariff, often just to get more data GB per month from a better deal.
Often the uSwitch deals have things like free Disney for 3 months, all depends on when you visit and luck that a promotion is active.
There is a tariff that appears occasionally on uswitch.com for around £5 but it's not always available.