I would send them an email or something asking why it doesn't work. It does mention in the blurb that the offer is available for new and existing customers.
Maybe they are charging you because according to the instructions you get the PIN through logging in on the above link. The PIN obviously proves you a re a VM customer.
After a quick look on their website it says contact Virgin Media customer services which just links to the help pages (not very useful at all). It also mentions Virgin Vault which seems to exist no longer.
I've highlighted in bold the reasons (I think) why it may not work below:
Terms and conditions
Incremental postage and packaging charges apply. Offer applies to 6x4" prints only during the promotional period as specified by Snapfish Offer is strictly one per customer only and is valid for Virgin broadband customers only. Offer is valid on single image 6x4" photo prints only. The offer excludes collage prints. Offer excludes free prints, prints credits and prepaid credits. Each additional prints purchased in the same order will be charged at standard rate. Monthly free prints offer expires at the end of each month and offer is only valid for a 12 month period. Offer applies to Snapfish customers with UK delivery address only. Offer cannot be used in conjunction with any other Snapfish promotion. Customers must sign up at the specified URL and follow the steps as described above. Snapfish reserves the right to withdraw or amend the offer without notice. Please allow additional 1-2 delivery days during this promotional period.