Your IP address is dynamic, but that doesn't mean it will change often if at all. Your modem gets it's IP address as a lease from the Virgin Media server. That lease will have an expiry time. Simply rebooting your modem doesn't reset the lease, indeed the server just reallocates the old IP.
You could simply power off your modem for the length of time needed to let the lease expire. However, Even if your lease were to expire, many areas are that busy that you would get the old IP re-allocated.
Probably the most reliable way of forcing an IP change is to change the mac address of the device connected to the modem, and then reboot the modem. That is relatively easy if you use a router where you are able to "clone" mac addresses of the connected computer cards into the router (and thus potentially change a small part of that address). Not so achievable if you use a direct modem to computer conection.
Don't confuse mac addresses of network device such as ethernet cards, with mac addresses of the modems themselves.
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