Cashback offers are normally offered by and administered by third parties like topcashback, I've never seen that offered by Virgin Media themselves so you'd need to check where you found the deal, and contact them. At the point of click through, these deals often look like the offer is from VM, but that doesn't mean they are.
It's like the cashback offers with mobile phones - you agree a deal through cheapofones4u for a 24 month contract with say Vodafone, and the deal includes cashback. Read the small print, and it is cheapofones4u who provide the cashback, under very strict conditions. Cheapofones4u are funding that offer by giving away some of the premium that Vodafone pay them for funnelling customers their way. But the cashback isn't part of your contract with Vodafone, and that contract with Voda will say nothing about cashback. If cheapofones4u go bust, or renege on the deal, you have no comeback against Vodafone. And so it seems likely that's the case here.
There's certainly procedures for have complaints escalated to the industry adjudication scheme, in my honest opinion you're on a hiding to nothing down that route unless you have a contract with VM that explicit states that you'll get £75 cashback.
