If I could add to that, I think one of the major elements of the petition system has been that it's an ongoing relationship with the person who signs. If a debate is selected, they receive a notification of when that debate happens. Signing the petition is not the end of your involvement with the process. You are notified and you're invited to then watch the debate online, which is how they get those viewing numbers. It's a way of having Parliament appear more responsive.
What Sir David told you was actually already inaccurate. The largest petition ever in the Westminster Hall system received six million signatures. It was debated on Monday. In the British practice, it's not a direct relationship between the number of signatures on a petition and then the debate. There's the intermediary body of a committee that can have some editorial control. They grouped one petition against Brexit with another in favour of Brexit and had it be debated in Westminster Hall. They had high traffic.
The other thing I would note is that it allows those members to take part, but the main element is the ministerial response at the end.