No, not everyone may sign the petition. But anyone can in a few clicks. The House of Commons claims to check the authenticity of signatures.
We rephrased your question: "Is it true that anyone can sign the petition against Brexit on the British government's website? Even French people, as can be seen on Twitter with tweets by French users who claim to have signed it?"
Your question alludes to the petition "Revoke article 50 and stay in the EU" on the British government's website, which has been signed more than 5 million times. You would like to know if these 5 million signatures are authentic. We can already tell you that at least three are false: ours.
Anyone can sign in just a few clicks
To sign a petition on the British government's website, the signer must be of British nationality or a UK resident. But the signer does not need to provide a copy of a passport or document proving a British address. It is enough to tick a box to declare: "I am a British citizen or a resident of the United Kingdom".
We were able to sign three times: the first, pretending to live in the English town of Warwick; the second from France; and the third from Honduras. We used throwaway email addresses to conceal our real identities. The form simply asked us to confirm our email address and click on a link to validate the signature.
The petition site, run by the British government and parliament, provides geographic data about the signatories, divided by country or parliamentary constituency. Result: 5.16 million signers were declared for the United Kingdom, 39 448 for France, but also 30 for North Korea and 59 for the Vatican. It is impossible to know whether these were really neighbors of the Pope or Kim Jong-un, or just jokesters pretending to live in the UK by finding a valid postcode in a few clicks on the internet...
Without elaborating, the petition commission claims to check the authenticity of signatures
On Friday the House of Commons's petition commission claimed, on Twitter and without providing details, that it was being careful to exclude fakes: "A few people have been talking about fraud and overseas signatures. As of this afternoon, approximately 96% of signatures on the petition were from the UK. That’s broadly what we’d expect for a petition like this. We don’t comment in detail about security measures. We use different techniques - automated and manual - to identify and block signatures from bots, disposable email addresses and other sources that show signs of fraudulent activity. We also monitor signing patterns."
We don’t comment in detail about security measures. We use different techniques - automated and manual - to identify and block signatures from bots, disposable email addresses and other sources that show signs of fraudulent activity. We also monitor signing patterns.— Petitions Committee (@HoCpetitions) March 22, 2019
When contacted by CheckNews the House of Commons press department says that the 96% figure concerns signatures declared from the UK, divided by the total number of signatures. In our exchange, we let them know that we had been able to vote several times using throwaway email addresses, obviously fake names like "Test Teste" and UK postcodes found on the internet or simply invented. Following our fake signature based in Honduras, we observed that the number of signatures declared for that country increased from 8 to 9. The House of Commons subsequently told us that petitions are subject to post-facto checks in order to correct fraudulent signatures.