Joe,
As a frequent user of Heathrow, I've never heard anyone say they are committing to their destination. I have heard plenty of people diverting when they don't have enough fuel for the given holding time/EAT. I've gone into my diversion fuel on a few occasions but it's nobody else's business so why declare it? Although Heathrow is far and away the most likely place to have to hold for, the Heathrow controllers do usually give accurate delay information. Conversely, I recently flew into a large European airport in a country noted for its efficiency. Fifteen miles from the IAF we were told to hold and expect a twenty minute delay. Just before entering the hold it was cancelled and we were given a tighter than normal approach (as were the other inbounds at that time, from what I could hear).