Well I bit the bullet and went for a nonrefundable rate at the Langham - I know my plans won't be changing except in the event of some unfortunate circumstance. I tried reaching the UK number but it was busy each time I tried, so I called the US HHonors folks back and this time the agent found the promo. It still gets quoted as $137 GBP per night, but for two nights including dinner. She says it does include taxes - which I guess makes sense since its nonrefundable, and they're charging me the entire amount now. Dinner by the way is listed in her records as TDH/Carvery, but she didn't know which restaurant. (TDH I later found out is table d'hote, kind of like prix fixe.
She gave me a king room with a request for an upgrade (HH Gold), even though the ad specifies twin/double. I know we'll be much happier in a king.
FYI - the agent had no info on cancellation insurance as listed in the ad.
Thanks again to Gus for alerting us to this deal - I hope I'm not the only flyertalker to take advantage.