You may find in the terms and conditions somewhere that if you buy an upgrade, even online, that purchase of same constitutes a new 12 month contract, even though you don't actually sign a document. Certainly was the case for me with O2 when I last upgraded.
Edit:
Here are the relevant terms and conditions from Meteor's site:
The Term of this Agreement shall be for a Minimum Period of 12 months from the date of connection to the Network. This Term and Minimum Period will be extended for a further period of 12 months following any upgrade of your Equipment, which will include the re-execution or new execution of terms, or your acceptance in writing of any new terms and conditions
It appears that the requirement for a signature to extend the contract is an optional extra.