Try the Customer Retention team. Dial the normal VM customer service number, then select options 1, 1, 4, 5 (assuming they haven't changed the menu - last one of those should be "thinking of leaving"), and that should connect you to the UK based customer retention team. They're usually pretty good and helpful in getting things fixed if you're at the point of leaving VM.
If you've been a customer for a while and have an old telephone password that doesn't meet the current minimum standard, you might be told that option isn't available, and you'll need to speak to the offshore call centre to get it reset - the call should be diverted automatically, can't say how quickly it will get answered. Last time I did it it was answered pretty quick. Then try again.