I had the same result when trying to do a GPR last week - they are no longer allowed to manually override the system. I sent in a complaint to BA and got a cut and paste response saying they understand why I'm unhappy with these changes, they'll pass on the feedback to the relevant team but they're basically not going to do anything. I would suggest anyone else annoyed by this change contacts them as well - if only so they can't say that no-one seems to be bothered by this.

On a separate note, having initially been told that a GPR wasn't possible on a wet lease flight, of which there are quite a few at the minute, customer relations have advised that it is possible.