You can easily launch your own political petition at http://petitions.pm.gov.uk/
The site has had more than 3 million signatories from different email addresses.
It is also interesting to look through the petitions already listed and to add your name to the ones you approve of. It may not be a legal vote, but it is heading towards Interactive Democracy.
Here's what The Times have to say about No.10's OnLine Petitions.