You should have been told the available dates which are available for you change to (not all days are available.) As long as you your bill is less than your credit limit (or you have no credit limit) you won't have your services cut off the next working day. You'd best discuss the matter with the billing team to inform them in advance of your money situation (not being able to pay on time this month.) That can be done via a call to 150 from a VM phone / 0345 454 1111 from any phone -> Option 1 -> Confirm your details -> Option 1 (Billing) then choose one of the appropriate billing options.
Or reach them via the Webchat from a desktop/laptop between 8AM to 8PM M-F /& 9AM to 5PM Sa: Account and Billing -> Understanding my Broadband, TV and Home phone bill -> Chat online. If the agents are busy,
______ Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.