For those intending to go and wanting to visit the vaults.
As you go in, you're given a numbered ticket and (probably) will see a line of people headed for the vault tour. The ticket is to be handed in as you get to the start of the tour. The number doesn't have any significance. If there's a long line they'll tell you that you can visit the exhibition on the ground floor and come back to the line. They won't tell you that if you visit the exhibition first, you lose your place in the line. It's not like the line in a doctor's surgery where they serve people by number.
As you go in, you're given a numbered ticket and (probably) will see a line of people headed for the vault tour. The ticket is to be handed in as you get to the start of the tour. The number doesn't have any significance. If there's a long line they'll tell you that you can visit the exhibition on the ground floor and come back to the line. They won't tell you that if you visit the exhibition first, you lose your place in the line. It's not like the line in a doctor's surgery where they serve people by number.