Upgrading to HS1 tickets

[EUC]

We're travelling on not SE high speed tickets. We wanted to upgrade at Margate, but the ticket office is shut today.
The platform staff said, not to worry — we could upgrade on the train. However, an hour into the journey, and there's no sign of a ticket person.
What do we do if we get to St Pancras without having paid for the upgrades?

 

[Watershed]

Then you pay the upgrade to the staff at the barrier.

There's no penalty for paying for the upgrade onboard the train or at the next available opportunity.

 

[youngpete]

They often check tickets between the Medway Towns and Gravesend on HS1 trains.