I submitted another claim for another night in the same week and had a claim approved. My cheaper rate was on amoma.com this time. There was only one room available so I got lucky they didn't sell it before IHG checked. It's one night at the Intercontinental Park Lane in London. Unfortunately it's just a Classic Room (they look tiny and old fashioned), but at least it's with breakfast.
I only hope I won't have to pay for it and claim a refund afterwards. I'd rather avoid the hassle.