Although they say when they sell you the contract it is for a Fixed term 12 or 18 months it's not fixed they can put it up annually , You should have got a email saying that it was going up and you had the right to cancel, But try getting through to them , Also if you had phoned to cancel they would have given you the option to press a button and get 6 months at a reduced fee.
Check your latest bill and the previous one for any discounts/promotional periods ending. Also check that you haven't been charged for any out of contract phone calls, eg out of hours or over 60 minutes, also for any movies you may have bought/rented. Any extra charges should be itemised on your bill.
If the increase is due to discounts ending then contact regrades or retentions and negotiate a new deal.
If you don't want to be tied up on the phone you could try the text messaging service. Just send a text with a description of regrade to 07533 051809. Your message will go into a queue and a representative will respond to you as soon as possible.It may take a few days.
You can also call 150 from a Virgin landline, or 0345 454 1111, options 1,1 and 4 (Changes to your package) butbest to call between 8am and 10am to avoid call queues, or late afternoon around tea time
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali