For nearly a year my broadband has been rubbish I pay 200mbps a month but I only get between 20 and 40mbps, that is when it works. Took Virgin Media to CISAS and won my case I was awarded compensation and Virgin Media were told to sort my broadband out but up till now nothing has changed. Virgins customer service is rubbish.
My guess is its a congestion issue and I too have suffered with the issue in the past, two years went by and a lot of publicity on the issue but not easy one to fix, in the end left went to a different provider.
Unfortunately it doesn't look like Virgin give a toss about providing minimum service requirements, they just let customers go if they're not happy.
Its a case of they don't have to give you anything, you just don't have to stick around and get nothing:)