1) Unless the people who sign the petition had not voted (or voted for brexit, in which case you could even see it as a x2 effect) you can't "make the difference" using these numbers as they doesn't necessarily show any sway.
2) Why would there need to be a pro-brexit petition if brexit won the vote? That is unless cancelling brexit suddenly becomes the way that decision-makers want to go for (and note that there are pro hard brexit petitions)
Disclaimer: I do not reside in the UK, and I no longer click on every article in the ever-flowing torrent of brexit-related news, so my knowledge of the ongoings may be outdated