You are not the account holder so can’t re-negotiate the contract.
You could get your ex to transfer the account which costs £20 but, I believe, still means a credit check which you may fail unless your credit health has improved sufficiently.
Your best option, I suggest, is to get your ex to cancel the account, or get it declared she has gone away and cancelled. You could then become a new customer with better pricing but again with the potential credit health issue.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.