Submission Conditions
An e-petition may freely disagree with the government or call for changes of policy. There will be no attempt to exclude critical views. Decisions to accept or reject will be made on an impartial basis.
However, to protect this service from abuse, e-petitions must satisfy some basic conditions.
To create or sign an e-petition, you must be either:
•a British citizen
•a resident in the UK (you normally live in the UK)
These are the conditions from the official Gov't website--servicemen are not barred from signing.