The HH number should show up on the computer folio at the check in desk. If they do not acknowledge your number verbally, ask if it is there. If it isn't, the agent should be able to do a "search profile" on the screen. Just be sure to let the agent know the address in London where the HH card is registered. Chances are there will be multiple members with the same name. You should be able to add your HH number to any reservation you have at any time, even at checkout.