dial 150 or 0345 454 1111 and follow the prompts to - 'thinking of leaving us' - then "technical issues" - dont worry about that they do much more - open 8am -10pm 7 days a week - should take you to a UK call centre
be aware any change to your package will start a new contract
My 100mb broadband and phone package went up from £32 to 46 per month. I called Virgin Media, three times. In between searched and spoke to friends on switching options. On my 3rd call, predictably more waiting time than talking, I decided to stay after negotiating. Downgraded the broadband speed and removed the phone that were used once a week.
I would persist, be nice and be very patient. It's worth calling to negotiate.