I'm in exactly the same position. Was more than happy to renew my contract at the agreed price but the activiation date came and went and like you am sick of the constant broken website. I'm not begging to continue to be a Virgin Media customer so I will vote with my feet as their customer service has gone rapidly downhill. My contract ends on 23rd May and I will be cancelling my contract IN WRITING BY RECORDED DELIVERY as I have read on here about too many people being unable to cancel sucessfully and continuing to be charged. I will also be cancelling my Direct Debit and any monies owed I will send by cheque.
Good Luck