Stayed there last thursday, and also they were not volunteering the breakfeast. When I insisted that this is one of the Gold-/Diamond-benefits, the lady took a look in a list and shortly after replied with "Sure, I'll add that to your rate".
When I got to the breakfeast-room the next morning, nobody was aware of it, but when I showed my gold-card, everything was fine.
In total, though, I did not like the Hilton Hyde Park too much. Too touristy, too loud and a little unattentive service. Stayed the night before at the Metropole, and that was simply a great experience.
Greetings - Dirk