Similar experience at the Gatwick Hilton. After some discussion, and a request to speak to the manager on duty, I was finally upgraded to a club room. However, they refused to give me access to the lounge, as 'it is not part of our upgrade policy, you got the upgraded room, goodbye.'
I guess the lounge costs the hotel money, but the room, if unoccupied, does not.
I won't stay at this hotel again, simply because of the attitude. I have other stays at different Hiltons over the next month, I wait to see how they act.
Tim