The current advice, gleaned from this forum and others, is not to order changes to existing packages online.
The most important reason is that you are unlikely to get the best price for your bundle using that route.
Best speak to someone at Virgin Media. Phone early morning and you will likely get someone in the UK. Always check pricing via the following webpage before you call:
http://allyours.virginmedia.com/html...e-builder.html
PS Remember to add the £11 phone rental charge into the price shown (it's in the small print just below the headline price).